Unpopular Used Foreign Cars: BMW!! The Yellowing Around the Gas Tank Cap is a Concern [How Far Can We Go with Polishing and Painting?] ☆Vol.20

BMW K75S 1987

In the year of the 50th anniversary of BMW's "M series," our BMW K75S was reborn with a "M" color scheme based on pearl white. Based on a model known for being unpopular, equipped with an inline water-cooled engine, we have refreshed the coloring, polished various parts, and performed essential maintenance, seeking a unique and complete form different from other K series models. The area around the breather passage of the gasoline tank cap had started to yellow, so we consulted with a professional paint shop.

Masking the inside of the tank with thick aluminum tape

The layout of the genuine BMW fuel tank breather passage might be the root cause of everything. After removing the cap, we covered it with thick aluminum tape to prevent debris from falling in and brought it to the shop. If it had been painted in the same color as the modern M series' black pearl, the yellowing might not have been so noticeable.

Modern four-wheeled M series models often have a black metallic pearl base color, rather than pearl white. If it had been black pearl, I think it would have turned out with a different atmosphere. However, for our generation, this color is just right.

The first FI engine model on a BMW motorcycle. Is that why it came standard with an aluminum tank?

I brought the K75 fuel tank to Dream Shokai, where I requested a full repaint, thinking "Let's consult first." Mr. Kojima, the representative of the workshop, immediately came up with an idea and started working on it. He is a fan of Honda Classics.

The vertical wall of the collar part that combines with the tank cap was also painted pearl white. Mr. Kojima proposed an idea to finish the paint at the edge of the tank skin and leave the vertical wall part as exposed aluminum to minimize the yellowed area.

With the aluminum tape masking I applied still in place, masking tape for painting was applied to the boundary between the tank skin and the tank cap. The paint around the tank cap was then removed so as not to damage the pearl white of the tank skin.

Mr. Kojima began peeling off the paint on the vertical wall by making a sharp cut with the tip of a design cutter on the paint film at the edge of the collar. It was a great decision to have used thick aluminum tape for masking to prevent the peeled paint flakes from falling inside the tank.

After peeling off the paint, several types of compounds were used to polish the exposed aluminum tank. As it was BMW's first FI model, they likely adopted an aluminum tank to prevent rust and avoid clogging the injection nozzles.

Starting the pre-polishing with non-silicone, non-wax compounds

Debris accumulated in and around the breather passage was blown out using an air gun. At this stage, the yellowed area was almost unnoticeable. Further finishing work progressed from here. Polishing was carried out with a polisher using various compounds.

Even looking at it this way, you can tell it's an aluminum tank. It was a good thing I thought ahead and masked the tank flange holes with thick aluminum tape. Mr. Kojima also commented, "This aluminum tape masking is sturdy and good."

The color tone changes significantly depending on the type of pearl mixed into the base color

Touch-up paint is being prepared to improve the finish of the pearl white paint's edge. Pearl white itself comes in various types, from bluish to yellowish. The tendency of the pearl mixed into the base white determines the different color tones.

Before applying the touch-up, the surrounding pearl white areas were polished with a no-wax (non-silicone) compound. The slight unevenness and jagged edges at the peeled paint line will be repaired with touch-up paint. Work is proceeding carefully, as these are visible areas.

After the touch-up is complete, the masking aluminum tape is removed. Because the adhesive side was facing upwards and the aluminum tape was thick and strong, no dirt or paint flakes fell into the tank. This procedure worked out well.

The dirt and peeled pearl white paint flakes that had accumulated around the tank cap were almost entirely caught by the adhesive side of the aluminum tape. The boundary between the applied tape and the adhered debris is clearly visible. It was good that the masking was done successfully.

Glad that no debris fell into the tank!! This is truly the benefit of masking

The collar part of the tank cap after repair. Since it is an aluminum tank, there is no rust inside the tank at all. This can be said to be a major characteristic of the first generation K series. I feel very sorry for the R series owners who are troubled by rust.

When the tank cap is attached, it looks like this. The part where the aluminum base is visible is the top surface of the tank cap collar. The paint on this part had yellowed due to reaction with the volatile gasoline from the tank breather. The yellowing should no longer be noticeable.

The "M" series is known as BMW's sports lineup. While I don't know if the M1000RR, known as a homologation model for the WSBK scene, overlaps with that "M" image in its design, the 2002 series I admired as a child also had unique color models called the "M line." They looked incredibly cool, and the higher-end 3.0 CS series also had "M line" models.

Did you know that the first model of the "M" line, featuring a pearl white base with three colored stripes, celebrated its 50th anniversary in 2022? In that year, I purchased a K750S, one of the less well-known models in the unpopular K series equipped with the first-generation vertical engine, featuring a 750cc three-cylinder engine. I decided to finish it with an exterior graphic inspired by the special color "M" series. It was purely a personal sentiment, but I wanted to recreate the "M" series in my own image in the year of its 50th anniversary.

Finishing it with a pearl white base made the yellowing around the gasoline breather noticeable. I was honestly disappointed. When I asked an experienced motorcycle enthusiast familiar with BMWs, they told me that white models of the popular R100RS series also often suffered from yellowing around the tank cap breather, just like my K75. It seems to be a kind of chronic issue. By the way, modern "M" series models use a black pearl base color, so if it had been that specification, I might not have noticed the yellowing. The breather passage is almost certainly the culprit, with the pearl white around the passage becoming particularly yellow.

Wondering "How can I fix this yellowing?" I took the tank to Dream Shokai, where I had requested the paint finish, and consulted with the representative, Mr. Kojima, asking, "What if we painted the collar part black to make the yellowing less noticeable?" Then...

"This tank is made of aluminum, isn't it? In that case, instead of painting, why not try exposing the bare aluminum first and see how it looks?" he suggested.

That made sense. If it's bare aluminum, it can be wiped clean and polished even if affected by gasoline. While discussing this, they immediately began the repair work. After attaching the finished fuel tank to the bike and assuming a riding posture, the yellowing around the tank cap, which used to bother me, no longer does. Now I can ride with peace of mind again! Ah, that's a relief.