A few things.. to summarize, I painted 2 areas, one with standard cheap rustoleum and one with the professional line from Lowes.
On the standard side, I said that I didn't do the right surface prep. I sanded, then dusted off with a rag. To do it right I should have sanded, dusted with air, then cleaned with a clean rag and mineral spirits.
When I am scratching the paint with my nail, two things are happening - I am digging my nail into the (soft) paint, and then I am lifting the paint off as I drag my nail down it. Like a razor blade. I do not think there would be any problem at all if I couldn't dig my fingernail through the layers of paint to get to the surface in the first place, so I personally don't place ENTIRE blame on poor surface prep. I can rub all day long on the paint and it aint going anywhere (to answer William Hall's question). I put most of the blame on the softness of the paint, after all I can dig my nail all the way through to the surface. If I couldn't do that, my nail would rub down the paint and maybe leave a small scratch. GRANTED, enamel may take a long time to dry, so I am not closing the book quite yet - But if the material was sufficiently hard, I would NOT be able to dig my nail through the layers to the surface and the paint would sit there happily doing its job.
On the professional side, I believe there was oil or something where the bubbling took place. I think when I prepped the area, I might not have spent much time with mineral spirits in that area where it bubbled up since I was focusing on the area underneath it (on which there is no bubbling, but which has a very uneven surface on the metal so it doesn't look good). Next time I'll scrap the mineral spirits and use an actual grease remover (I have a can of stuff just for this purpose, but I tried with mineral spirits). The professional side, after only 10 days, is harder to scratch than the standard side. I think, as above, that this is a combination of better surface prep (sanding + dusting then mineral spirits cleaning) and a quicker drying paint. This is a good sign for the professional side, as well as lending some weight to the idea that the standard side surface wasn't prepped right. Admitted!
One more thing - the C38 quote above was about TREMCLAD. For instance he says he can not scrape the (unthinned) paint off his can with his fingernail, BUT I COULD. And one should not argue that the paint can (the paper label, the metal itself, whatever) was prepped differently for a can of tremclad and a can of rustoleum. I do believe, with all the regulations here in CA and in the USA, that there could be a significant difference between Rustoleum and Tremclad. What it is, I don't know.
Several people in this thread have stated that they have x,y,z, painted with rustoleum and it's now as hard as a rock. So perhaps these scratch tests shouldn't have commenced for a few more weeks, but the lesson here is it will take time to dry, and take time to properly prepare the surface.
I am certainly not questioning that the technique is a good one. The whole reason I experiemented in the first place was to see how nice it can turn out, and not having much of any paint/body experience before this, I am really learning as I go.
