The Ford Barn - View Single Post

Kevin in NJ · 09-02-2010, 12:17 PM

Keep in mind I was very specific to talk about the recoat window of the epoxy primer.

First the idea of recoat windows.
The older paints like lacquer would melt the previous layer and chemically bond with the existing paint (way simplified here).
Modern 2 part or 2K paints have a chemical reaction take place where the molecules like up to become one giant molecule that is your paint. Once the reaction happens you can not restart it, it is done. While it is reacting (curing) the paint has openings in it to which the next layer can grab onto and bond (again very simplified). Once out of the recoat window the paint is very smooth and the next layer would have nothing to grab on and bond. To get the next layer to bond you would need to first sand it to create scratches on the surface.

I will try to address the primer/ surfacer uses.

A primer is put on to be sanded to help level the surface. If the sheet does not talk about recoat windows it is likely because the primer is designed to be sanded. So you would be doing finish sanding then the top coat.

If you have a patchwork of paint/ bondo on the surface as you some times get as you sand to level the surface. This patch work of surfaces may be visible through the top coat if they are not first sealed with a surfacer.

The primer sealer is usually put on thicker to fill and thin to be a surfacer. You can see in the K36 sheets they give you the option to reduce or lay it on thick. Marco indicates that they have you use different hardeners today. They do this to meet the VOC when sprayed rules. You can probably still use the old directions where you add reducer. The old directions sometimes make life easier unless the EPA catches you violating the VOC rules. I was using the old mix ratios with the Dupont URO a bunch of years ago and found a huge difference in getting the primer to lay nice, but I am quick to add that I am not a very good painter. When used as a surfacer you try to get a nice glossy coat and spray the top coat without sanding. The other option is to spray a light coat and give it a light sanding to get the dirt and dust level like us hobbyists tend to manage to get in our paint.

09-02-2010, 12:17 PM	#16
Kevin in NJ Senior Member Join Date: May 2010 Location: South East NJ Posts: 3,398	Re: Featherfill high build primer Keep in mind I was very specific to talk about the recoat window of the epoxy primer. First the idea of recoat windows. The older paints like lacquer would melt the previous layer and chemically bond with the existing paint (way simplified here). Modern 2 part or 2K paints have a chemical reaction take place where the molecules like up to become one giant molecule that is your paint. Once the reaction happens you can not restart it, it is done. While it is reacting (curing) the paint has openings in it to which the next layer can grab onto and bond (again very simplified). Once out of the recoat window the paint is very smooth and the next layer would have nothing to grab on and bond. To get the next layer to bond you would need to first sand it to create scratches on the surface. I will try to address the primer/ surfacer uses. A primer is put on to be sanded to help level the surface. If the sheet does not talk about recoat windows it is likely because the primer is designed to be sanded. So you would be doing finish sanding then the top coat. If you have a patchwork of paint/ bondo on the surface as you some times get as you sand to level the surface. This patch work of surfaces may be visible through the top coat if they are not first sealed with a surfacer. The primer sealer is usually put on thicker to fill and thin to be a surfacer. You can see in the K36 sheets they give you the option to reduce or lay it on thick. Marco indicates that they have you use different hardeners today. They do this to meet the VOC when sprayed rules. You can probably still use the old directions where you add reducer. The old directions sometimes make life easier unless the EPA catches you violating the VOC rules. I was using the old mix ratios with the Dupont URO a bunch of years ago and found a huge difference in getting the primer to lay nice, but I am quick to add that I am not a very good painter. When used as a surfacer you try to get a nice glossy coat and spray the top coat without sanding. The other option is to spray a light coat and give it a light sanding to get the dirt and dust level like us hobbyists tend to manage to get in our paint.