Epoxy Primer and Topcoats

[BradK]

New to the forum and had a couple questions on top coating using a single stage polyurethane over SPI 2.1 VOC epoxy sprayed over fresh aluminum. I am building a Van’s RV-12 experimental airplane kit and am going to spray some control surfaces and wings that have been completed using the SPI self etching 2.1 epoxy primer. Will epoxy primer be an acceptable base for a single stage polyurethane topcoat or will i need to apply a SPI 2K primer coat between the original epoxy 2.1 and a single stage top or finish coat? Appreciate any inputs.
Thanks,

 

[dhutton01]

I don’t think SPI epoxy is self etching. My understanding is that epoxy chemistry is such that it cannot be self etching.

Don

 

[BradK]

I don’t think SPI epoxy is self etching. My understanding is that epoxy chemistry is such that it cannot be self etching.

Don

Hey Don, Thanks for the kind reply to my question. As i read in SPI’s 2022-23 Tech Manual it states the following for Epoxy Primer 2.1 (VOC, i live in Calif, no bad stuff allowed), “This epoxy eliminates the need for an acid-etch primer“. Since i will be using this on brand new sheet aluminum here is how i’ve seen the steps for this process.
1. Wash aluminum using warm water with Dawn dish detergent mixed in using a “maroon” colored ScotchBrite pad.
2. Wipe clean with dry clean towel and let dry.
3. Wipe cleaned metal using lint free rag with SPI’s Waterborne Wax and Grease Remover and allow to dry for 45 minutes
4. Mix 2.1 VOC Epoxy Primer at a 1:1 ratio and allow a 30 minute induction time before spraying the mixed product.
I watched a fellow aircraft builder on YouTube using this product with the steps as I described above and he swears the stuff to be tough as S#$T and cannot scratch it very well while even using a screw drive after it cures. This is how i found this product. Typically builders would have to perform a chemical conversion on the aluminum using alodine, very toxic and no way to collect chemicals and water when rinsing using that process thus SPI Epoxy saves this step. Still not sure about shooting top coat onto the epoxy though?
Brad

 

[elwood]

Yes, it would be an acceptable base. No need for 2k if no dents or work is necessary. I use epoxy exclusively on my projects that require a long enduring paint job. 2k is mainly for making the surface completely flat. And filling in scratches and minor imperfections.

 

[Dean Jenkins]

Yes, it would be an acceptable base. No need for 2k if no dents or work is necessary. I use epoxy exclusively on my projects that require a long enduring paint job. 2k is mainly for making the surface completely flat. And filling in scratches and minor imperfections.

Agree. Just watch the recoat time window.

 

[Barry]

I don’t think SPI epoxy is self-etching. My understanding is that epoxy chemistry is such that it cannot be self-etching.

Don

Epoxy is different; most people think of an "acid etch primer" that does have a small amount of acid in it when self-etching is brought up.
Dtm primer is also different, but the dtm is made by a percentage of one polyol at less strength than the main one and, of course, has various corrosion additives added.

 

[Lizer]

SPI epoxy is a soft epoxy (a dried puck is the consistency of a powerball) so it indeed should scratch easily…even cured, hence the reason it sands so easily.

 

[Barry]

SPI epoxy is a soft epoxy (a dried puck is the consistency of a powerball) so it indeed should scratch easily…even cured, hence the reason it sands so easily.

The complete cure on my epoxy is about 90 days sitting in a shop.
Here are two unprepared or clean cans sprayed as a check when Lizer had a fisheye problem.
The white can, I did 4 coats wet, with no flash time trying to run it.
For the black, I did six coats the same.
It has been setting since, and as you can see, the knife barely shows in the black and not the white. It has been a month or so
It's not fully cured yet and gets harder as it sets.
Remember: I did a 4inch thick puck test, and it took almost a year to cure.

 

[BradK]

Thank you all for your great replies to my question as to using SPI 2.1 VOC Epoxy Primer as an undercoat for single stage polyurethane topcoat. My plan is to shoot the airplane parts and then store them until it is time fir assembly. Since i will be out of SPI’s recommended window for re-coat (7 days), i will hit with 320 grit and re-shoot another coat of Epoxy primer and then shoot a single stage polyurethane finish color.
Brad