I asked DC Plastics for a price for shipping to the UK and any advice for spraying.
“Shipping would be approximately $55 for one fender. I don't recommend painting fenders if you are going to ride the bike.”
 

Yamaha painted the fenders in the year 1975 with quite succes. 46 years later, the quality of plastic primer and paint is much better. Dont see any problem riding a painted fender .

I asked DC Plastics.....
“HDPE plastic is very hard to get paint to adhere too. When the fender flexes, they chip easily.”

 

I'd hypothesize that when Yamaha made the fenders they were using LDPE; Low Density Poly Ethylene which doesn't hold up as well over time as HDPE; High Density Poly Ethylene, but does take paint better due to a more open structure.

A quick google search says "LDPE is softer and more flexible than HDPE. It also has a lower melting point (115° C) and is more transparent. Compared to HDPE, it is more likely to crack under stress.

HDPE is rigid and durable and offers greater chemical resistance. Its higher melting point (135° C) allows it to withstand higher temperatures than LDPE. Its more crystalline structure also results in greater strength and opacity of the material."

That chemical resistance is probably a large part of the paint adherence issue.

Hi all - I’ve hopefully just purchased a DT400 mudguard (fender) which is straighter than the one I already have. It will arrive next week.
It will need lots of preparation (the limited pictures I have of it show it quite rough looking).
So, has anyone got any good experience of a sprayer / bodyshop in the uk that has successfully prepared/painted one of these recently?
I live near Reading.
It would be good to take it to someone who has experience of spraying 1970s plastics rather than the more modern plastics.
Many thanks....

Found some pictures, im my folders, comparing stock fender with repro from Thailand.
Found some pictures, im my folders, comparing stock fender with repro from Thailand.
  

Super excited this morning. My new (ie, old and rough) front mudguard arrived. Seller wasn’t sure what bike it was off, but I took a punt on it. Anyway, it’s the correct one (DT250/400 twin shock).
I’ve stripped the paint and I’ll take it (along with all the other ‘silver’ parts on the bike) to get them repaired as necessary and painted up.
I’ve just got to decide which silver to paint them! I need to confirm a uk name/code. I’ve seen a few uk names/codes mentioned in previous posts.... Any up to date suggestions warmly received.
Thanks again......
 

I sprayed mine with no problems.

Plastic primer from Halfords after a rub down with wet & dry - I think 800 and then sprayed with a rattle can Ford Diamond white.

The paint has taken well with no chips or peeling so far - 500+ miles on the road - not off road.

Hi all, just a quick update. I used a heat gun to get the bends out of the front mudguard. Then I gave it to a car body spray shop (along with all the other silver sprayed parts on the bike) and the result is that it looks brand new.
To be honest, I think the car body spray shop did a bit more work with the heat gun, and lots of work with filler/sandpaper etc etc.

Nice!