Morris Minor Owners Club

What's the most likely cause of tiny (barely visible) cracks like these appearing in the cellulose primer-filler on Fenchurch's bonnet? Is it poor adhesion, or could it just be that I sprayed it on too thickly and it shrunk as it dried? The cracks are mostly around holes, but there's also some in the bottom of a concave section. Do I have to strip the whole thing down to bare metal and start again or can I just sand the cracked areas down and spray some more primer on?

1st have you body filled over the area that has cracked?...what type primmer are you using? has there is a few diff type....you should use etch primmer 1st then use builder primmer and then top coat.......flaten the primmer after each one has it gives a better finsh and also did you clean the areas properly b4you started with panel wipe?....you should be able to flaten it down a bit then use builder primmer which will fill those cracks, also what % mixture have you done 50/50 is what you need for primmer and to be mixed well and also the right temp, so not to much moisture in the air

No filler, but most of the bonnet has the original paint under the primer except where there were rust spots, which I sanded down to bare metal, treated with Jenolite, and sprayed with Zinc primer. I'm not sure if the cracks are all over ex-rust-spots but it's possible. I suspect I didn't do a very good job of the Zinc primer stage because I hadn't got the gun set up right and it went on a bit "dry," and I didn't etch-prime over the Zinc primer (possibly a bad idea). I used panel wipe. I didn't flat down after each coat of primer (didn't know that was necessary). The primer is mixed 50/50. Not sure about the temperature but it may have been a bit on the cold side.

Alex, you could be suffering from a reaction of the new primer with the old top coat. Are you sure that the original paint was celly or had it been resprayed. The symptoms you describe show shrinkage of your new primer which can be caused by low temperature, moisture in the air (celly sucks up moisture), or you have applied the coats to wet, or applied them to close to each other ie you should wait for a flash off time normally 10-15mins to allow the thinners to evaporate.
There is no need to flat between coats, but I'd suggest that you strip off all paintwork and start again, whilst you have only got to the primer stage.
Apply a mist of etch allow to fully dry, mist a coat of celly then follow with two more wet coats alowing for a flash off time as above.
In general the trouble with priming over existing paint is that the new will reactivate the old,softening it up, this can then shrink and dry at a different rate to the new paint. Also avoid putting filler onto existing paint as this will also sink, if not in primer then definately in top coat.
A good product you can use is Davids Reface a high build filler primer normally used to fill imperfections or pin holes. this should be applied and flattened with a 240 disc, then prime over this with your celly of 2K . It should provide a non sink layer thus helping eliviate the potential problems listed above.

Thanks. I'm pretty sure the old paint is Cellulose; most of it is original. There's an area on the bonnet that has been touched up before but that's not where the cracking is. Low temperature, too wet, and not enough drying time all sound like definite possibilities.

After sanding down some of the cracked areas on the bonnet I have a theory as to what caused it. The zinc primer has more extensive cracking than the cellulose primer over it - only some of the cracks have made it through to the surface - and the cracking is worst where the zinc primer went on quite thick. I reckon either the zinc primer wasn't dry all the way through when I sprayed over it, or the thinners in the cellulose primer reactivated it, and it redried over the next few days at a slower rate than the cellulose primer, which caused it to keep on shrinking after the cellulose over it had hardened. To be on the safe side I've decided to completely strip the bonnet to bare metal and do it again without a zinc primer coat this time.

Worryingly a crack has appeared in the primer next to the driver's side guttering on the front roof (I think the zinc primer went on quite thick there too because I was trying to get it to cover the whole inside of the gutter section).

i believe zinc primmer is based for bare metal other wise the rust will still start underneath the paint etc so unless u want it to rust i would put it on

Alex, only spray a thin coat of zinc, directly followed by your celly primer, I'd be tempted not to etch onto the zinc. Or just etch and prime. I'm sure you are aware that celly is a great absorber of moisture, so try and make sure that the room in which you paint is both warm and dry, only apply thin coats of both etch and primer and give each product 24 hours to fully cure before either applying the primer onto etch or flatting down the primer. When you have wet flattened allow a further 12 hours atleast before top coat, again making sure that the room stays reasonably warm and dry.
It sounds as if your findings over the cracks bear the truth.
Good luck

I've cleaned it back to bare metal and etch-primed it (a single very thin almost-translucent coat), and I'll leave it until tomorrow to do the cellulose primer. I was a bit wary of zinc-priming it again after what happened the last time, even if it was my fault for putting it on too thick and/or not leaving it long enough to dry.

In my experience you are best to use the same manufacturers products from start to finish as you are more likely to get reactions otherwise.
I personally would have used a good 2k primer then top coat with etch primer used on bare metal areas. Celly tends to pickle rather easily what's below it if it's not really compatible. I've even heard folk getting bother when using jenolite or similar as a rust treatment.
You may be better to forget the zinc and just use the primer if you're getting a reaction.

Pete