fresh epoxy

I need a quick answer on this question: will freshly mixed epoxy adhere to cured epoxy? Thanks much!!!

 

If the epoxy is completely cured you have to (lightly) sand it first. Unless you used peelply. If you don't sand it might adhere but not very good.

Good luck, Hans

 

Yup.

You need a bit of a texture to the fully cured part; light sanding works well (if you used peelply, pull it off immediately before you start the secondary layup, and it'll take the surface contaminants with it and leave a perfect texture. If you take the peelply off and then wait a day or a week, you'll have to clean the surface again, so leave it on until you're ready to bond).
And it has to be clean- no oils, waxes, grease, etc. I like to give the existing part a good scrub with acetone, once the sanding dust has been vacuumed up, to get rid of any remaining contaminants.

 

I don't believe in lightly sanding well cured epoxy. Epoxy bonds, relying on a mechanical adhesion (what you're trying to do) requires a fair amount of "tooth" for a good bond. Don't even think about anything more then 100 grit, with more aggressive grits being preferred, depending on how much finish work will be necessary.

In other words, if the surface you're bonding to is a structural element and out of sight, where finish work will be minimal, then use 24 to 40 grit and scuff it up good. If the part is to receive a fine finish, make it smooth with 100 grit, do your bonding, then build up the smoothness after the bond with progressively finer grits in the finish coats (of whatever).

 

40 grit would be ideal, yes.... thanks for pointing that out PAR A key piece of info that Hans and I missed.

I say "light sanding" in the sense of "don't grind away into the substrate". As opposed to heavy sanding- last time I told workers that the part needed heavy sanding for secondary bonds, they ground away at it until the Kevlar fibres started coming off in little fuzzballs.

Anything over 120 grit, IMHO, is for finish sanding and has no business coming out until you're prepping for paint or varnish....

 

Thanks to all for your invaluable help! Much appreciated!

 

yes

clean with alcohol / methelated spirits / water and soap

sand it

clean again

sand it lightly again - vacuum
clean again

clean again

clean again with clean cloth

 

If it is freshly cured West System epoxy, all you have to do is scrub with a 3-M pad and water to remove the anime blush.