I just sandblasted my inner door structure of my 69 firebird and am going to epoxy the bare metal so my question is this, do i have to scuff the sandblasted metal with 80 grit so the epoxy will bite or can I just spray the epoxy over the sandblasted metal, also I was hoping to spray the cowl and both inners together but that would mean leaving some metal bare for about a month, my shop right now is about 45 degrees and the humidity is almost none existent, would that cause a problem for rusting?

I put epoxy over sandblasted metal all the time. if the temps are in the 45 deg. range your epoxy probably won't cure it kind of goes dead at that temp. what dose the info sheet say.
Jay D.

Get the metal temp up to 70 degrees (check with infrared thermometer) before spraying and then maintain the temperature in your booth/garage above 50 degrees (absolute minimum) for the next 24 hours.
Remember to keep your epoxy warm also since spraying cold epoxy on warm metal will cause things to cool down too quickly.
I try to maintain 60+ degrees in the booth overnight after spraying epoxy.

'68 Coronet R/T wrote::goodpost:
Get the metal temp up to 70 degrees (check with infrared thermometer) before spraying and then maintain the temperature in your booth/garage above 50 degrees (absolute minimum) for the next 24 hours.
Remember to keep your epoxy warm also since spraying cold epoxy on warm metal will cause things to cool down too quickly.
I try to maintain 60+ degrees in the booth overnight after spraying epoxy.

Thanks for the info, I use ********** epoxy primer and I'm well aware about the temp to spray their product, I usually turn up the boiler to 70 a few days before i spray...but my original question is can i spray it directly over sandblasted metal or to I need to hit it with 80 grit to make sure the epoxy bites to it, the sandblasted metal seems pretty smooth also I'm using glass bead. Also how long can I leave my metal bare would a month cause it to rust?
Thanks again

it depends on your media profile really, if youre in doubt error on the side of caution and sand it with 80,

in my opinion i wouldnt leave bare steel or any other metal sit longer than 24 hours before coating.

Dave's correct, I was thinking you were using sand, glass beads even the heavier grade leaves a smooth profile. I use a 30-60 grade of sand and epoxy seems to stick to it good. 30-60 is about as fine as you can get in sand and has worked good for me. i also use dune sand from our local sand dunes, but only use it on unimportant stuff. I worry about salt content but it doesn't seem to be a problem.
Jay D.

Be careful using beach sand. As the crystalline turns to dust it is harmful to breathe. I haven't seen a fab shop use silica based abrasives in a while. At the very least if you are using it have a real respirator and not just a dust mask. Silica is bad news.

https://en.wikipedia.org/wiki/Silicosis

I use glass with 40-60 sieve (Potter's "C") and I can get it abraded enough for primer or steel impregnated coatings to bite. If something is steel and really rusted I'll use aluminum oxide but it could pit or damage some panels if it's too aggressive. If you are using the glass from HF or Northern Tool they advertise as "80 grit" (pro quality glass is sold using sieve sizes not grit) it's about a 70-120 sieve or "mesh" and probably closer to 100 grit than 80 grit. Like the guys say hit it with some coated abrasive and you should be fine.

Jay D.

Don't use Sand it contains Silica and while you may be using a breathing Mask your neighbors may not be. Silica is wicked bad for the Lungs and doesn't take much to affect a person lungs, seriously. Like Micro grams is all it takes to get a case of Silicosis.

Glassblast doesn't contain Silica is finer and very affordable https://www.glassblast.com/
I pay $11-$12 a bag for 100 lbs buy it by the pallet save more.

There are many others on the market that don't contain silica too

micro- (µ-) 10-6 1 millionth

The current OSHA permissible exposure limit (PEL) for respirable crystalline silica (quartz) is 100 µg/m3 as an 8-hour time-weighted average (TWA) [29 CFR** 1910.1000]. The NIOSH recommended exposure limit (REL) for respirable crystalline silica is 50 µg/m3 as a TWA for up to 10 hours/day during a 40-hour workweek [NIOSH 1974b]. This REL is intended to prevent silicosis. However, evidence indicates that crystalline silica is a potential occupational carcinogen

So you think sand dune sand from the ocean has silica? i'm thinking about going to the dunes with my fourwheeler today probably going to ingest a lot of sand.
Jay D.