Hello-I am a relative newbie to the site but read a lot of the forums and find them very helpfull.Recently I started the finish work on my 1940 ford ltruck.The truck was blasted with walnut shells except for the two doors which were in new condition-ie did not need any bodywork.The doors were sanded with 80 grit and then 240 grit and then shot with PPG 2k primer.The truck sat from Sept until Feb 17 in a heated garage with bodywork complete and all in primer.The truck was movedd to an unheated area of the garage for 3 days in Feb.When I went to get the truck to work on again the primer and all other paint on the doors had peeled and a lot had fallen off leaving the doors with shiny like new metal showing,the sandblasted areas of the truck appear to be fine.Using patience and fine sandpaper I was able to determine the paint on the doors was 1.red oxide primer2.black factory-enamel?3.grey primer-enamel? 4.Flat black enamel(paint job in the 60's.5.PPG grey urethane 2k primer.An interesting thing I noticed was that the original primer had turned a nice chocolate brown color on the metal side of the door which I suspect was caused by some sort of chemical reaction.I am guessing by not blasting the old paint off the doors I set myself up for this failure.Has anyone ran into a similar problem-the paint shop I got the primer from said the PPG urethane 2k would be ok over the old paint but I am thinking he was way wrong.Any insight would be much appreciated.Tks Gord

 

Typically oem paint that is in good shape can be effectively painted over...but, every once in a while a chemical reaction occurs....and the paint just peels right off...typically beginning to show signed anywhere from 24 hours to a couple of weeks. The you just take it to bare metal & start again.

 

Thanks for the info but the truck has been in primer for 5 months with no signs of lifting.It will be easy to complete stripping of the doors as almost all the paint has fallen off.I would still like to understand if the urethane primer caused the old paint to lift.Why was the reaction triggered by moving the truck to an unheated environment I talked to paint guy yesterday and he says it was not the primer as he paints over old finishes often if the paint is good and has never had such an issue.Gord

I would still like to understand if the urethane primer caused the old paint to lift...

IMHO, It probably didn't.......assuming the paint & primer were both compatible. In the world of chemistry, there are things that happen, reactions, etc. As an example (one that is easy to visualize)......when we look at chemical protective material, there are 3 ways that a chemical, when applied can damage the material...

Penetration- goes right through the pores of the material
Permeation- structurally the material is fine but when a chemical is applied to one side, a reaction occurs on the opposite side
Degradation- material just falls apart.

IMHO, it was definitely a compatibility issue, but is could have been cause by a variety of things from the oem paint, primer &/or paint....contacting each other (even via vapor/molecular leve) and creating a "synergistic" effect...meaning the 2 + 2 should =4, but when these tow or three chemicals mix, 2 + 2 = 7

 

Todays paint products use very strong reducers, and a lot of times will actually re-flow the old product. So you can sand very thoroughly a old factory surface and use a urethane surfacer over it and it will re-flow it and make it like it was never sanded. Reversible substraits i.e. lacquer products need to be completely removed, every paint manufacturer that I'm aware of will tell you this in the technical manuals. Also I would recommend nothing finer than 80 grit on bare metal surfaces. When stacking primer coats the mills build up and needs a good mechanical scratch to grab on to finer than 80 is in a sense polishing the metal because steel is a lot harder than the paper. Keep in mind if a primer is not a DTM, (direct to metal) you will need to put down an etching primer or you might get adhesion issues. This is just my opinion not saying this is what happened, without seeing it it is very hard to say.

 

I agree with postmortum....it is very difficult to tell without looking at it.

That said....urethane surfacer's and the reducers therein could potentially change and "oxide", causing it to lift the old paint.

 

What is the PN of the 2K primer you used? Best to consult with the technical data sheets, but some 2K urethane primers are not direct to metal (DTM) and require a DTM substrate that is compatible. Most guys will go with an epoxy and then the 2K high build. I only point all this out, as your bare and prepped sheet metal is suspect to delamination if the 2K is not DTM and you sprayed directly over bare steel.

On the doors, it is hard to say what was on there, and I tend to strip things to bare metal to insure there are no other hidden surprises. Paint products aren't cheap, and leaving on an unknown product it just asking for compatibility issues, as you have found. I would venture a guess that you have some Lacquer based products that the solvents in the 2K have reacted with, causing the Lacquer products to soften and delaminate. Likely the issue was not as noticeable in the heated shop until the doors were put in the cold, the metal shrank ever so slightly, where the bottom layer lost any surface tension that was there previously.

Going back to your 2K product, check the data sheet to see what preparation is required for a substrate or surface prep to insure the rest of your efforts haven't been in vain..