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Time window to spray next product after sanding

#1
Hello, I have read or maybe heard that after sanding 2K primer, you must spray the next product whether it is more 2K, sealer, or base within 24 hours otherwise the 2K "heals" and fills in the sand scratches. Is this correct? I am slow and it takes me a while in order for me to be ready for the next step (sometimes on the order of months)

So my questions are, is there a time window that after one product is sanded that the next step needs to be sprayed,
1. Bare metal sanded with 80 to epoxy?
2. Body filler on top of epoxy sanded with 180 to more epoxy?
3. Epoxy sanded with 180 to 2K?
4. 2K sanded with 600 to epoxy mixed as sealer?

Does it depend if the prior product is fully cured like epoxy at 20 days? If once the product is fully cured and will not shrink, you can apply the next product at any time (weeks to months) after properly sanding?
 
#2
I’ve been looking for an answer for question #4 and I’ve seen on other sites about other products that there is no window but on the SPI 2k directions it states that after 24hrs the primer needs to be scuffed before sealer. I’m assuming this is the correct way?
 
#3
Any bare metal should be primed immediately after sanding. I've seen some tech sheets say within the hour, but most shops settle for same work day. Sometimes for a hobbyist that will mean stripping 1-3 panels at a time, that's just the cost of doing business. All the other stuff, the rule is no later than next day in my shop. No use playing with possible adhesion loss down the road. Basically, if you're not going to recoat it soon, don't sand it.
 
#4
I'm curious... why does it matter if something was sanded 6 months ago or yesterday? I get if you are in the recoat window, you will have chemical adhesion, but outside of that it will just be mechanical(?) adhesion. Isn't an 80 grit scratch the same now as 3 months from now?

Not challenging, just curious.
 
#5
I'm curious... why does it matter if something was sanded 6 months ago or yesterday? I get if you are in the recoat window, you will have chemical adhesion, but outside of that it will just be mechanical(?) adhesion. Isn't an 80 grit scratch the same now as 3 months from now?

Not challenging, just curious.
Crash’s reply is specific to bare metal the reason for not leaving bare metal sanded for too long is rust, the OP question was specially about 2k primer being sanded and left awhile before spraying reduced epoxy, I’m also curious about this since I’ll be doing my work at home on weekends. Is it ok to leave 2k primer sanded for a week or 2?
 
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#6
From what I have read, no expert here, but alot of research on the forum.

The reason it is recommended to scuff 2k after 24 hours is due to shrinkage. If you spray 2k say today, sand tomorrow, and then wait a week to do something with it, that is an example where the 2k might have some shrinkage after sanding and now you might have an inadequate sanding scratch for adhesion.

If you were to spray 2k, let it sit for a couple weeks, put it in the sun a day or two, and then sand. At this point alot of the shrinkage will have occurred and your sanding scratches will stay better.

In both cases it is quick easy insurance to scuff it with a scuff pad if you are past 24 hours since it was last sanded. You can also get any corners or crevices that weren't blocked with a scuff pad at that point.
 
#7
From what I have read, no expert here, but alot of research on the forum.

The reason it is recommended to scuff 2k after 24 hours is due to shrinkage. If you spray 2k say today, sand tomorrow, and then wait a week to do something with it, that is an example where the 2k might have some shrinkage after sanding and now you might have an inadequate sanding scratch for adhesion.

If you were to spray 2k, let it sit for a couple weeks, put it in the sun a day or two, and then sand. At this point alot of the shrinkage will have occurred and your sanding scratches will stay better.

In both cases it is quick easy insurance to scuff it with a scuff pad if you are past 24 hours since it was last sanded. You can also get any corners or crevices that weren't blocked with a scuff pad at that point.
That sure sounds like a reasonable reason to me. Thanks
 
#8
That is kind of what I was thinking might be the case. I often block my epoxy down, but then sometimes it is months until I'm able to get back at that panel. Generally I always go over everything with a gray pad, I'm a bit paranoid.