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Last year I removed the metal plate from below the windshield and re-sprayed after stripping back to bare metal. At the time it looked like a perfect finish, given I prefer electrics and mechanicals to bodywork.
After a few weeks cracks started to appear and a year on it looks a mess.
I'm going to start again with new primer, paint and clearcoat all from the same vendor this time to avoid any reaction between the layers.
Looking for any other suggestions on what I did wrong so this doesn't become an annual event?
I would remove the cover and get it back to bare metal and start again, that usually happens when the primer isn't allowed to flash off and solvents in the base colour and laquer re-act.
another thing is applying the paint too thickly – which makes the surface of the paint dry too fast and not the underside. When you re-coat the solvents in the paint shrink and this starts the wrinkling.
I think you nailed it with the comment about choosing the same pain system for all the layers. I'll throw in a "follow the direction" hint too. Sometimes I get a bit in a hurry, especially onsmaller parts, and don't let the underlayers fully cure before adding the next. I can't tell the color of your car in the reflections. If it's a solid color then it was a single stage paint when shipped. No clear coat. Meanwhile, most repaints seem to be two stage these days, mostly because there's so much forgiveness available when it comes to getting the final texture just right. A local owner at our last place had a black car that had some respray done on one side, and those panels were fading and foggy within a year, and had some noticeable checking (as yours shows) within a couple years. The car was sold, and the new owner invested in a very complete prep and respray to bring the finish back towards original.
Every once in a while I accidentally fancy myself a good painter. Turns out I'm not. I've done many that are "OK" but only a few that would pass serious inspection without extra work. Brings me to the thought that it's often worthwhile turning work like this over to someone who does it every day. Our local paint and body places offer specials where they will do small repairs and reshoot a PU bumper cover for under US$150, including R&R. I'm guessing that the cowl cover there would be a bit less. Might be worth an ask-around.
Thanks for the comments, I'll try thinner coats and leave each to dry longer. If it cracks again I'll get it done by someone who has a clue, and stick to doing the hidden bits myself !