Sanding Epoxy "Outside" the Window

M

milanomaroon

Promoted Users
So a little background - several years ago after completely stripping my car I sprayed 3 or 4 coats of SPI epoxy primer. I blocked this with 180g dry sandpiper and sprayed another 2 coats of epoxy primer. The blocking and next 2 coats were within the 7 day window. I knew I would have to stop at this point for awhile but wanted to protect the bare fiberglass from contamination.

So, a year and a half later, I had some time and blocked these last 2 coats of primer with 320g dry. After talking to Barry I realize I should have used a coarser paper and will need to block again, spray 2 coats of epoxy, and then get ready for sealer, base, and clear. A couple of questions I have are:

1. Do I need to use 180g with a guide coat to block before applying the next 2 coats of epoxy? Just lightly sand with a hard block? Can I use a soft block with guide coat?

2. After applying the 2 coats of epoxy then sand starting with 320g through to 600g? or 800g? Would also use a guide coat. I am using a dark color with a small amount of metallic (5 grams of extra fine per 8oz) so I am not sure how fine to go.

3. What happens if I exceed the 7 day window? After waiting a day or two before I start sanding, then blocking again with 320g, 400g, and finally 600g, cleaning and masking the car off I expect to be close to exceeding my "window". And that's if I don't have any interruptions.

4. If I use a reduced epoxy sealer can that be applied after the 7 day window? Would it help to re-scuff everything if I am outside the window prior to sealing?

5. I would prefer to paint everything at once but perhaps I could work the doors and hood separately from the body and paint at 2 different times.

Thank you all for taking the time to help me. I'm pretty new at this so any ideas you have would be appreciated.
 
If you are confident it is straight as you want it, no need to block. You are just looking for adhesion. Something like these work great. http://www.sunmightusa.com/products/flex-sanding-sponge. No need to guide coat either, unless you just want to. 600 final sand, no finer, especially when sealing. If you are slightly past the window after final sand, a light scuff with a scotchbrite would be fine before sealer. Epoxy sticks to itself really well.
 
texasking said:
.You are just looking for adhesion. Something like these work great. http://www.sunmightusa.com/products/flex-sanding-sponge. No need to guide coat either, unless you just want to. 600 final sand, no finer, especially when sealing. If you are slightly past the window after final sand, a light scuff with a scotchbrite would be fine before sealer. Epoxy sticks to itself really well.
Click to expand...

Am I understanding this correctly? If I was ready to go to paint on my year and a half old epoxy (which is at 320g now) I could sand to 600g, apply sealer, and base/clear?

Is the 7 day "window" only an issue if I want to apply more epoxy primer or apply base without using a sealer?
 
imo the 7 day window is for applying more epoxy only. if you sand it then applying a urethane 2k primer or single stage i would give it no more than 4 days. basecoat i try to never go more then 24 hours. with base i always shoot a reduced coat of epoxy under my base maybe 15-30 minutes prior. i mix it 1:1:1 and just spray it like an adhesion promoter. 1 piss thin coat just to make the base stick really well then i dont really have to worry about recoat windows. i will shoot it like its my first coat of base.

if your ready to go and you get your epoxy to 600 grit, 400 on da with a soft pad would be better, then i would have no issue shooting a sealer coat over it then go right to base BUT you need to be really careful. you need to be sure you get the sealer coat on 100%. no missed spots or light edges. you also dont want to spray it too dry and you get poor adhesion. spay it too wet and you'll get craters. basically yes you can do it but it leaves no room for error. you could do 2 sealer coats 1:1:1 then wait maybe 2-4 hours, denib if necessary and goto base. that might be better. epoxy is the best primer there is but its a finicky animal and needs to be done right. im sure barry will always error on the side of caution with someone that doesnt spray the stuff every day.
 
Last edited:
The issue is your year and a half old epoxy will not give any chemical adhesion to what is sprayed over it. It is relying on mechanical only. 180 scratches will give stronger mechanical adhesion than 320 or 600. Epoxy will stick really well to itself within the window or sanded with 180 if completely cured.
 
Thanks to both of you for your replies. Needless to say this is very confusing for me with my limited experience.

texasking said:
The issue is your year and a half old epoxy will not give any chemical adhesion to what is sprayed over it. It is relying on mechanical only. 180 scratches will give stronger mechanical adhesion than 320 or 600. Epoxy will stick really well to itself within the window or sanded with 180 if completely cured.
Click to expand...
It sounds like anything finer than 180g is risking adhesion. Which tells me the safest course is to sand with 180g, reprime, sand to 600g and make sure I apply sealer within the 7 day window.

And Jim C it sounds like your solution would work but I am not sure if my skill or experience level is good enough to apply the sealer properly and I don't want to find out in 6 months or a year that it wasn't.

I'm OK with repriming and blocking again. It will give me another chance to see if everything is straight and more importantly it should prevent an adhesion failure later on. I just need to see if I can get it ready within the window or decide to paint in parts.
 
If you reprime and have then sanded with 600 then I don't see the need for sealer. You're ready to go straight to base. When I do my 'seal coat' I spray one coat of epoxy unreduced, wet sand 600, and then it's ready for base.
 
Lizer said:
If you reprime and have then sanded with 600 then I don't see the need for sealer. You're ready to go straight to base. When I do my 'seal coat' I spray one coat of epoxy unreduced, wet sand 600, and then it's ready for base.
Click to expand...
Using a sealer always gives you better adhesion than mechanical scratches do for basecoat. If the epoxy is in the re-coat window then it's not as big a difference but if it's outside the window I personally think it's mandatory. Doing like Jim says and shooting epoxy 1:1:1, over the substrate, then applying base will give much better adhesion.
 
I appreciate all of the information you guys are providing. It really helps and I think I know now what I need to do. But what happens if I re-block with 180g, spray 2 coats of unreduced epoxy, and start to block again. If I don't get completely finished before 7 days are up what do I do then?

Am I making this too hard by wanting to use only epoxy for the primer?
 
milanlmaroon said:
Am I making this too hard by wanting to use only epoxy for the primer?
Click to expand...
Yes, IMO.
I know some folks here block in epoxy.
But most, especially less experienced ones (like me) find it much easier to block primer surfacer. That is what it is made for.
-Coat bare surface in epoxy.
-2k primer surfacer for blocking, with guide coat. As many rounds as it takes to get flat with 180 or 220.
-One more coat of 2k to fill in scratches, sand with 400 or 600.
-Reduced epoxy sealer (optional but a good Idea) De-nib if needed.
-Ready for Base or Single stage.

**If recoat window passed, scuff as per tech sheet and reshoot.
 
Big difference between year and a half old epoxy and 10 day old epoxy. If you go over the 7 days, and are using epoxy sealer, it will be fine. The 7 day window is for recoating without sanding. With sanding, I think you're good for 30 days inside, no sun. I applaud you for using epoxy only. It is more of a pain and time consuming, but the results are worth it on something you plan to keep for a long time, IMO.
 
There are a lot of variables involved based on perfect conditions if epoxy is sprayed
1:1 with NO reducer added.
Remember, mils with epoxy are everything!

Mixed this way, body filler, and 2k primers can be sprayed with no sanding for seven days.
For the next 7 days, you must sand with 180 to apply filler or primer.
If spraying epoxy over epoxy without sanding, you have 14 days.
Anything after 14 days, you must sand the epoxy and epoxy.
Remember this is a general guideline, and yes have heard many times of restro shops sanding 3-week old epoxy with 180 and applying filler or 2k primer, but they are pros, and they can tell real fast if the filler or primer has full adhesion or not when sanding.

As a sealer, insurance shops will over reduce for speed, but the restro shops seem to reduce 1.1. And add 10 to 25% reducer and time to ad base coat varies as depending on the base used, I tend to give different answers after I ask a 100 questions.
Here is what I do know over reduced, you have a safe window shooting base, reduced 30% or less 2 to 3 days is safe, SS 7 days.

So many variables I feel like a politician giving these general guidelines.
The catalog is about time to order more, and im going to try and do a better job clarifying the epoxy guidelines.
 
You must log in or register to reply here.
Back
Top