How can you use a water based basecoat over a porous material in that case?
In the past 7 years I’ve been a painter we’ve never had issues with panels being left in primer for a length of time. Some lads like to get their cars in on an evening, repair a section and prime it, it then gets left for weeks or months before they get chance to paint it. Often panels come in primer from the factory, they could be sat there for months before they get shipped anywhere, and all of the primer isn’t removed or baked before priming, it’s not necessary.
Silicone is a b*tch! The stuff they use to shine tyres up and then they come in for a refurb is the worst! Just can’t get rid of it no matter how many times you panel wipe them.
Ash
Ash, I know what happens and I hear you, primer will be and is used longer than it ought to and often with no apparent ill effect but it's never recommended and can lead to paint issues in the future - i.e. resprayed cars with oodles of issues bubbling and lacquer peeling off while the original paint work is still 100% is a common enough sight.
I occasionally do things like this too - it's cheating in my eyes, where I might cut a corner or do something too quick or incorrectly to get the job done and did I get away with it? Yes I did. Example, I painted bare MDF drawer and door fronts I made last week, I spray paint as much as I can as I have all the kit and gallons of common paint colours in different finishes and types, 1K, 2K base coats, lacquers etc. and I do this as I can get a commercial grade finish on things. So I went to mix up my primer only to discover that I had used most of it on something I had forgotten I had even painted. I was in a mood and wanted these things painted fast - so I found a tin of varnish - thinned it a little, fired it into the gun and painted all the MDF with the stuff. It worked a treat, MDF sealed, left it 24hrs, de-nibbed it and hit it with a topcoat of satin 2K RAL 9016 - how does it look - MINT - 2 tiny spec of dust that I won't even try and buff out as they are low down and won't be seen. As a practise, if I was a £50K + kitchen manufacturer this would not really be acceptable? It would be slapdash and incorrect, but I did it and got away with it - durability I think will be fine, the the adhesion to the varnish I think will be fine and being indoors I don't expect to see any flaking issues.
As for replacement panel delivery paint, as far as I am concerned that coating should be removed completely before starting your own painting - on cars it is the only way to achieve the OEM paint system.
You are a painter so as you will know, primer is really just a leveller and adhesion promoter. Now if it was an impervious coating, like that of the substrate you have just painted i.e. metal, there would not be much point in using primer as paints adhesion to metal is far less than to wood for example. Primer coat is only going to be about 30microns thick, so when the basecoat or solid topcoat solvent (water or chemical) soaks through the primer it will not cause a problem unlike say a piece of stone or MDF where it could soak in inches! Put a piece of kitchen paper on the counter and spray it with a little water - lift it up and the counter will be almost dry, saturate the paper with water and lift the paper towel, the counter will be wet and probably wet beyond the edges of the paper tower as it is saturated and cannot take any more - the same happens at low level with a primer coat. It is this penetration of the primer due to its porosity that makes primer such a good substrate for painting but it also means it can be contaminated easily.
As for your waterborne base coats, during drying once the basecoast films it creates a semipermeable membrane which the solvent can pass through by reverse osmosis, this is what allows the drying process to continue after the paint has filmed over, same happens in waterborne, only that it's water and about 12% solvent. Ideally in a basecoat, you obviously don't want too much solvent left before lacquer to avoid popping etc. However in the case of a waterborne basecoat (which will leave the much thinner layer than solvent based basecoat) the water should be fully flashed off before you you lacquer or you risk die back which is why cross draught booths are better for water. It is low humidity which helps dry waterborne rather than needing warmer temperature, albeit warmer temps will allow the air to hold more water and thus dry the paint quicker, but air movement alone will dry water well. However in the case of your solvent based basecoat the solvents don't need to be quite as dry, say 15mins for solvent rather than 45 for water, because any small remaining volume of solvent will dry through the lacquer.
