51 BMW 1973 R75/5 Preparing Fenders & Tank for Painting

After the successful first engine start, I turned my attention to repainting the fenders, tank and Windjammer II fairing.

I wrote up how I repaired the Windjammer and stripped the paint.

This write-up focuses on hand stripping the paint from the fenders, fixing cracks in the fenders, and preparing the tank for bead blasting. I will post separate writeups on setting up a temporary paint booth in my workshop, and how I prime, remove low spots and dings and apply base coat (color) and clear coat to the parts.

I am bead blasting the tank to avoid using caustic and messy paint stripper and to get a nice surface for the primer to bond with. The old paint held up well as the bike was always garaged. But, it’s showing the affects of wear and tear from the many years of being ridden. The original owner had the Windjammer II fairing painted to match the bike. And until I looked closer, I thought the paint on the bike was original.  Now I think the entire bike was repainted when the fairing was added.  One clue is the sticker on the top of the rear fender is missing and the other is the inside of the fenders only have a light misting of Moza Blue paint and no primer. So, I made the decision to sand off the paint and primer down to the bare plastic rather than rough up the existing finish and prime and paint on top of it.

Starting Condition

Here are some of the pictures I took of the condition of the fenders and tank.

Front Fender Gouge and Scratches

Front Fender Gouge and Scratches

Front Fender Rock Dings

Front Fender Rock Dings

Rear Fender Cracks & Gouge

Rear Fender Cracks & Gouge

Rear Fender Cracks

Rear Fender Cracks

Rear Fender Scrapes

Rear Fender Scrapes

Rear Fender Gouge & Pinstripe Detail

Rear Fender Gouge & Pinstripe Detail

Crud Inside Rear Fender

Crud Inside Rear Fender

Rear Fender Gouges

Rear Fender Gouges

Tank Scratches & Dings

Tank Scratches & Dings

Tank Pinstripe Position & Scuffs

Tank Pinstripe Position & Scuffs

Tank Dings and Pinstripe Layout

Tank Dings and Pinstripe Layout

Pinstripe Layout

The pinstripe layout is likely not original. But based on what I found out, they are very close to the original size and placement. Note that the BMW factory pinstripes were hand laid and therefore vary from bike to bike.  Consequently there is no single “this is correct” layout on the parts.

Rear Fender Pinstripe Measurement

Rear Fender Pinstripe Measurement

Rear Fender Pinstripe Detail

Rear Fender Pinstripe Detail

Tank Pinstripe Near Kneepad Measurement

Tank Pinstripe Near Kneepad Measurement

Tank Upper Edge Pinstripe Measurement

Tank Upper Edge Pinstripe Measurement

Tank, Lower Edge Pinstripe Measurement

Tank, Lower Edge Pinstripe Measurement

I found information from Craig Vechorik at Bench Mark Works about the pinstripes (Search the “Online Store” for “pinstipes”, go to part # “accessory 035A–” and click the technical article on stripes link.  Here is what Craig says about the stripes on Earles fork models:

  • Narrow stripe: 1/16 inch
  • Stripe gap: 1/8 to 5/32 inch
  • Wide stripe: 1/8 inch
  • Distance from edge of fender: 1/4 inch

Based on what I measured in the pictures above, these stripes closely match the dimensions of the earlier /2 models. So, I’m going to use a 1/8, 5/16, 1/16 pinstripe stencil when I reapply them. The stencil allows me to paint the stripes on but without the 1,000 hours of experience needed to be a professional.  At least that’s my plan right now.

Stripping Paint From Fenders

The fenders are made from a resin and I don’t want to use paint stripper on them.  It could damage them and possibly be absorbed preventing the primer from adhering. So, it’s time to break out the sand paper, blocks and water.

Paint Removal Tools

Paint Removal Tools

I have a small wood block, a large hard rubber block and a soft rubber block. After some experimentation, I chose 150 grit paper to remove the clear coat and color coat. Then I use 180 or 220 grit to remove the two layers of primer exposing the white resin surface of the fenders. Either of these grits are acceptable for surface preparation for the primer I will be using.

Rear Fender Paint Stripping Progress

Progress After Two Hours of Work

The picture above is what I accomplished after 2 hours of sanding. It took about 9 hours to finish stripping the rear fender and 8 hours to strip the front. The rear has quite a lot of surface area and several rounded surfaces where the tail light bracket mounts while the front has the raised center ridge with tight corners at the front and rear of the fender.  I have to be careful in these areas so I don’t damage them so it takes time and patience.

Rear Fender Outside Stripped

Rear Fender Outside Stripped

My Stripping Procedure

I tried dry sanding with 3M 150 grit and wet sanding with a special 3M synthetic 150 grit to remove the clear and base color coats. The 3M synthetic worked much better. The final sanding down to the resin used either wet/dry 180 grit I got at the auto paint shop (not 3M brand) or some 3M wet/dry 220 grit I had. The 3M paper worked better and stayed sharp longer the paint store paper. To wet sand, I dip the paper (wrapped on a block if I’m using one) into a container of water and then sand. I use a spray bottle to wash off the removed paint if their is a lot of paste starting to form. I rinse the sand paper in a container of water when the paste of removed paint starts to build so I keep the paper for loading up with removed paint/primer. I use lots of newspaper on my bench to soak up the water and sludge. When the top layer gets soggy, I added a new dry layer and keep going. When I get to the end of a sanding session, I roll up the soaked messy paper and toss them. Wet sanding paint and primer off parts is messy, so dress accordingly. I find that putting some masking tape on your fingers keeps them from getting sore and abraded as you work. Use some good hand lotion after you end a sanding session as the water and paper will dry out your fingers and palm. I start by removing the paint along the top of the raised trim on the edge of the fenders with a small wood block. I wrap the paper on the block and put the edge into the crease to remove paint and primer from the top of the trim and the side of fender panel. I tilt the block back and forth on the edge removes the paint on the rounded top of the raised edge to avoid flattening it. This is time consuming work. Then I use the small wood block to remove the paint and primer from the side of the raised trim and along the bottom edge.

Using Block on Fender Ridge

Using Block on Front Fender Trim Ridge

There is always a little bit of primer left right in the crease where a raised section joins the rounded area of the fender. The picture below shows some in the crease along one side of the raised center trim on the front fender.

Primer in Crease of the Front Fender Center Ridge

Primer in Crease of the Front Fender Center Ridge

I fold the paper to get a sharp edge and then gently sand the primer to remove it.

Fold Sand Paper to Get Into Crease

Fold Sand Paper to Get Into Crease

Sanding With Folded Paper in the Ridge

Sanding With Folded Paper in the Ridge

Primer Gone from the Crease in the Raised Ridge

Primer Gone from the Crease in the Raised Ridge

On the front fender, the holes for the lower fender brace are raised. I fold the paper several times so I have a stiff rounded edge and then carefully sand the paint and primer from the sides of the raised hole to avoid flattening them.

Front Fender Bottom Bracket Hole is Raised

Front Fender Bottom Bracket Hole is Raised

Carefully Sanding Edge of Raised Hole

Carefully Sanding Edge of Raised Hole

After I’ve removed all the primer from the trim areas and and raised holes, I use the soft rubber block to sand the larger rounded sections. I use a circular motion now to remove any linear scratches I may have created when I was sanding along the edges of the trim and raised sections. I find that getting the paint off the raised trim always removes some of the paint and primer from the surfaces next to these. That’s why I sand the large rounded sections last so I just remove the remaining primer and do not over sand the areas that already have the paint and primer removed.

Use Flexible Block to Sand Rounded Section

Use Flexible Block to Sand Rounded Section

Front Fender Ready to Wash

Front Fender Ready to Wash

Last, I sand the inside of the fenders to remove the crud and old paint. I pay attention to the edges of the fenders and remove any paint and primer and make sure the edges don’t have any burrs, gouges or rough spots. Here are the two fenders after being completely stripped.

Stripped Front and Rear Fenders

Stripped Front and Rear Fenders

An Alternate Stripping Procedure

I was talking to Kent Holt at Holt BMW who had looked at this material when I first posted it. He mentioned he used a special Xacto blade to scrape the paint in a sharp radius or tight corner of a part.  He sent a couple to me with the paint order.  I used this by holding the edge nearly vertical and then pulling it toward me. I chipped the primer and paint off very easily.

Xacto Blade for Scapping in Tight Corners

Xacto Blade for Scapping in Tight Corners

Kent also said he used some wood carving “draw knifes” he had picked up some years ago and they also worked well to scrape the paint from plastic parts.

I tried the Xacto blades and they worked very well removing the paint in tight radius corners without laborious sanding .  That got me thinking. I don’t have any draw knifes and the ones I priced were “pricey”. And, I am also stripping the parts for the Grey Ghost, my R75/”S” clone. When I painted it (the first time I tried painting) I screwed up the paint job by not using the correct primer. So I will repaint it in R90/S Smoke Silver when I paint the R75/5 in Monza Blue because it takes some time to get setup for painting, so I might as well paint both bikes at the same time. I decided to try a utility knife blade as a scraper. I find I remove the primer faster, but it is hard to hold the thin blade and my fingers get cramped. I found a plastic blade holder at my local Ace Hardware. It comes with single edge razor blades, but these are too thin and bend too much making them gouge the plastic. I find that using a piece of cardboard folded in half with the utility knife blade inserted inside the fold makes it thicker and fit tightly into the groove in the holder so the utility knife doesn’t twist in the holder.

R75/6 Side Cover After Removing Failed Paint

R75/6 Side Cover After Removing Failed Paint with Utility Knife Scraper

I used the utility blade to scrape the paint from all the parts on the R75/6. I estimate I saved about 30-40% of the time it took me when sanding the R75/5 parts down to the plastic.

Here is how I did one of the R75/6 side covers. Drawing the blade toward me starts shaving the primer off. After the initial shaving starts, it is easy to pull/push the utility knife blade to speed up removing the primer. The angle of the blade makes a difference; too vertical and the blade just skips over the primer and too horizontal and it cuts too deeply. After a little practice, the right angle becomes second nature. I change the utility knife blades often as it requires a very sharp edge to work well. When I don’t get much primer to shave off and I start pushing harder on the blade, it’s time to use a new blade.

Using Utility Knife Blade to Scape Primer from Side Cover

Using Utility Knife Blade to Scape Primer from Side Cover

R75/6 Side Cover After Scraping with Razor Blade

R75/6 Side Cover After Scraping with Razor Blade

As shown above, I scraped down to the point where there was a little of the last primer layer left. Then I use 220 grit wet paper to take that off and get the part ready for the new primer.

R75/6 Side Cover Final Sand with 220/180 Grit Wet

Preparing the Tank for Bead Blasting

I remove the gas cap, knee pad and enamel badges from the tank. The gas gap has a hinge pin that I drive out with a drift. Note that the hinge pin has serrations on one end.  You want to drive the pin out so the end with serrations comes out first.  I tap on one side of the pin with a drift and it doesn’t budge, so that side has the serration. I use the drift on the other side of the pin, and when I tap on it, the pin moves.

Location of Hinge Pin in Gas Cap

Location of Hinge Pin in Gas Cap

Use Drift to Remove Pin in Gas Cap

Use Drift to Remove Gas Cap Hinge Pin

Pin Coming Out of Gas Cap

Pin Coming Out of Gas Cap – Serrated Side First

Gas Cap and Pin

Gas Cap and Serrated Hinge Pin

Tank Filler Neck Detail

Gas Tank Filler Neck Detail

The knee pad is loose at the top on the right side and I can gently peal it off the tank. The left side is more tightly stuck, but carefully lifting one edge and using a paint scraper to loosen it from the glue and working aroung the edge, I get it off in one piece. The enamel badges screw on and have a rubber gasket underneath. I gently loosen the gaskets with a small screw driver and get them off in one piece. The badge below is damaged. I bought a new badge to replace it.

Knee Pad, Enamel Badge, Gasket and Screws

Knee Pad, Enamel Badge, Gasket and Screws

Here’s the tank ready for bead blasting. I took the R75/5 and R75/6 tank to BFN Industries, the folks who did my powder coating, to do the bead blasting.

Tank Ready for Blasting

Gas Tank Ready for Bead Blasting

Bead Blasted Tank

I got the tank back and it looked like this. I asked if they had primed the tank, but they hadn’t. The blasting creates a roughened surface that gives the same appearance as a grey primer.

Tank After Bead Blasting - Looks Like It Has Grey Primer On It

Tank After Bead Blasting – Looks Like It Has Grey Primer On It

Kent said he uses 180 grit dry paper to knock down the sharp edges of the metal before priming it. You don’t want to wet sand the steel or it will rust. This is how the tank looked after sanding.

Tank After Sanding with 180 Grit Dry NOT WET!!!

Tank After Sanding with 180 Grit Dry NOT WET!!!

Repairing Cracks in Plastic Parts

The fenders get cracks around the holes where they mount to the frame, and the rear fender always gets cracks where the tail light assembly mounts. These need to be repaired before priming.

Cracks Near Rear Fender Mounting Holes

Cracks Near Mounting Holes

First, I use a burr on my Dremel tool to grind out the crack to the bottom and round out the end of the crack to remove the stress concentration.

Using Dremel Tool and Burr to Grind Out Cracks

Using Dremel Tool and Burr to Grind Out Cracks

Cracks Ground Out

Cracks Ground Out

I use Fiberglass Resin to fill in the ground out cracks. When it is mixed up with the hardner, it’s pretty runny.

Fiberglass Resin Used to Fill Ground Out Cracks

Fiberglass Resin Used to Fill Ground Out Cracks

I paint this on with a small brush I have and let it set over night. I will sand this down so I’m not too careful about applying the resin. Usually the resin won’t completely fill the groove. I’ll fix that with some body filler later.

Rear Fender Tail Light Hole Cracks Filled with Fiberglass Resin

Rear Fender Tail Light Hole Cracks Filled with Fiberglass Resin

Here is the rear fender tail light holes and front fender mounting holes after I sanded out the Fiberglass resin with 220 grit wet.

Rear Fender After Sanding Fiberglass Resin

Rear Fender After Sanding Fiberglass Resin

Rear Fender After Sanding Fiberglass Resin

Rear Fender After Sanding Fiberglass Resin

Front Fender After Sanding Fiberglass Resin with 220 Grit

Front Fender After Sanding Fiberglass Resin

3 thoughts on “51 BMW 1973 R75/5 Preparing Fenders & Tank for Painting

  1. Pingback: 1973 BMW R75/5 Rebuild: Prepare Fenders and Tank for Painting | Motorcycles & Other Musings

  2. Hi You are obviously a perfectionist which I admire but in the world I live in I would rub down any rash repair the cracks as you have done & cover with a enough coats of primer filler & get a smooth great finish. I restore cars N motorcicles. Why take the original paint off that’s been there for years. In my book if it ain’t broken why make a dogs breakfast out of it.
    This is not NOT a criticism of your excellent article. You know that I have praised you before N I praise you for this. I’m just putting another perspective.
    Keep up the excellent articles. However it appears you haven’t covered the starter motor if I’m not mistaken
    Cheers
    Ralph

Leave a Reply

Your email address will not be published. Required fields are marked *