Humidity is always a deal breaker when painting.
When you spray paint, the drop in pressure in the paint plume causes a drop in temperature, this in turn cools the air/paint stream and the surface it is sprayed on (this is how refrigeration works). The drop in temp promotes moisture to condense from the atmosphere on to your job. Paint won't stick to a film of water!!

I would be pre-warming parts to about 80C and then painting. Avoid humid days, never paint when actively raining.

Sand your parts with wet and dry and use etch primer to key into the metal part.

Follow these simple instructions and the paint will stick like shit to a blanket!!

Ohhhh noooooooo!!!!!!! WTF
Such a nice finish on the paint and this happens now???
As my painter recomended me to leave the original paint as a base.I sanded all a bit and de-greased all.Washed with water and detergent and let in the sun to dry and get some temp.Then painted with Krylon(similar to VHT) ceramic engine paint.
For the cilinder and head I used other brand as a primer.Rust oleum caliper paint(450º cels 900 far)and then a thin spray of Krylon on top.As stated on the instructions second layers rigth after a few minutes or wait at least 7 days to dry.Or it will peel of like raisin skin if you over paint next day.
Also remember that cilinder,head and cover where sand blasted too.But my paint sticked like shit to a blanket!!
No rest of gas,solvents must be on the surface to be painted.

I wonder if its because its magnesium? i used VHT on bare metal casing & never had any flake at all, but that was alloy not magmesium, has it come off on any other parts Sven?

EDIT:

Im patting myself on the back here lads (pat, pat, pat)

Did a quick search & found this article on painting magnesium.

Magnesium metal is especially difficult to paint because it must be deoxidized and pretreated first. In addition, only specific types of paint should be used to coat this type of surface or the finish coat will chip and peel rather quickly. If you need to paint magnesium metal, you need to know the proper preparation steps and the correct materials to use or you could have disastrous results

Mezo.

Mezo,

good point... magnesium could be the issue (did your article say how to paint magnesium though... "you need to know the proper preparation steps and the correct materials to use or you could have disastrous results"... well yes, it did, soooooo... any guidance how to get out of the situation - anyone?)

It cracked almost over night whereas the valve lid and the stator lid seem to hold up all right (presume they are standard aluminium?)... might use Eric's primer concept next time round (the lids don't get that hot, so a standard primer might be sufficient). And will heat the bastard up as well! And will only paint in sunshine from now on... that should do, shouldn't it!?

better luck next time...

Just read Mezo's link for painting magnesium - sounds pretty complex, so had another idea I might try: Paint the lid with POR15 and then spray satin black over the top. POR15 sticks like hell and I cannot see that it wouldn't work on magnesium lids either. Might ponder that option over night.

None of my engine parts were painted with heat proof paint, only the exhaust got some pot belly black.
Regards Eric

Id go with Eric's recommendation Sven, etch primer.

Mezo.

Or give it a whirl with a can of magnesium wheel paint.....

Sven!
I was just reading the parts catalog in german for the 1983 34L.......... and to my surprise your seat is listed as "Einzelsitzbank"!!!!!!!!!So indeed your bike has a solo seat and listed with part number.......and your indicator holder.......
You own a standart german model 34L for the year 1983

Well, another weekend gone...a bit more progress!

First I started with the tank and the suggested "drown in caustic for days" exercise. Didn't have a large enough tub, so the wheelie bin had to do.



(no idea why the pictures aren't showing the right way round... they are correct in Photoshop and then not here... tried 3 times and still not right - just turn your head... )

Poured two kgs of caustic into it and left it in there for two days... didn't do much though, I guess the water wasn't warm, which should have aided the process, but how do you heat half a wheelie bin for two days. After pulling it out, some paint came off but the main white paint stuck solid - sort of: Was able to chisel the main layers off... now the tank looks like to has the army treatment - camouflaged! Will go for paint stripper again or get it blasted... sick of stuffing around with it. It is the last bit to get ready for painting. Most other parts are now done.



Even the stubborn magnesium clutch lid seems to have worked. Etch primer and painted in the sunshine has done the trick.



While I was painting I also used a sophisticated robot...



Also finished off the fork legs and triple clamp - looking great with clear lacquer on them - after using the scotch brite and degreaser trick they have come up fantastically well.



All engine parts have had a few coats of engine VHT paint (black satin) and they have come up well - just need to be cured in my wife's oven (doh!). I love the removal of the taped off areas after painting... makes it all look new and shiny and professional looking. Also did the exhaust with VHT super high temperature paint...hmmm, feels very rough and takes on finger prints easily, but will see if it lasts once the engine starts up.



And ordered some more parts from boats.net: Mainly bearings and oil seals, but also some other bits like fork boots (splashing out on OEM parts). Should be here within a couple of weeks.

Then all I need is the front mud guard and hand guards (believe both are still available through Yamaha?), a gasket kit and new oversized piston rings, maybe new tires, the tank and headlight cover painted and then the assembly starts - exciting times!

Can't wait for the Easter weekend... 4 days of time, might start assembly earlier....