Looking for some advice. I’m looking to getting something sandblasted and then etch primed straightaway but can I weld straight onto etch primer or grind it back ? Thanks
Don't use etch primer.
It is porous and old school.
A much better option is to get it primed with epoxy which will keep the steel in good condition if it is to be kept in primer (normal if it's a restoration or bring part time)
I also agree it's better to remove the paint before welding.
If you are spot welding a thin coat of zinc weld-through primer shouldn't do much harm.
If you are butt welding, remove the paint
All the spot-welded seams I've taken apart on un-repaired cars have the OEM ecoating inside the seams.
If you are using a spot welder, in my opinion, you are better to have a bit of protection inside the seam and the possibility of a slightly less good weld.
In a few years there is no point in having perfect welds if the rest of the seam is rusting out.
If you are butt welding or plug welding get rid of the paint at the weld site.
With plug welds a spot weld drill bit is a great tool for removing the paint.
Most of the weld thru are zinc silicate so will produce porosity if over 45 microns thick (usually are at corners) but theres one called Bloxite which is Aluminium based (made by Tempil) and it can be welded through.....but its expensive....I once ran some trials involving Lincoln, Harland and Woolf and Hempel Paints.....we made various weld thru primers....I wanted one that was Nickel based....it worked slightly better that the Zn base but it was too expensive.....In the end they used the 45 micron limit and Innershield, without grinding clean.....for a while anyway (then the H&W welding engineer changed)
Depends on the time delay really. You have already had the best advice, but if it will not be for long, you could spray any aerosol paint that you have lurking, then fetch it off with thinners fully before welding. Then you must epoxy prime on completion and before you do any filling or stopping.