Painting over media blasting

[Dcal]

If it's blasted with new sharp media it will cut the cleaning cloths to pieces and leave loads of fibers stuck to the steel that are impossible to remove.
I'd just blow it down with clean air and put the epoxy on.

It's always best to clean off as much muck and dirt as possible before you start blasting.

 

[ajlelectronics]

If it's blasted with new sharp media it will cut the cleaning cloths to pieces and leave loads of fibers stuck to the steel that are impossible to remove.

Even so, the surface can be sprayed with solvent and allowed to drip off. Making no attempt to clean it is just ludicrous, but I do understand that there are "armchair" experts here as well as professionals.

 

[Dcal]

Even so, the surface can be sprayed with solvent and allowed to drip off. Making no attempt to clean it is just ludicrous, but I do understand that there are "armchair" experts here as well as professionals.

In industrial applications where the coating system could be designed to last in excess of 20 years in very corossive environments, there is normally no cleaning after blasting, just apply the coatings usually within 4 hrs.
If the test results shows it needs cleaning, say because of elevated salt levels, it will be cleaned, reblasted and retested before applying the first coat.
There will be quality procedures in place to ensure you are blasting a clean surface and not spraying oil or other contamination onto the surface.

With the correct procedures there is no need to clean after blasting, but everyone is free to do as they please or adjust to suit their particular situation.

I am no expert, but I have been involved in the painting of enough marine structures to know the basics.

 

[ajlelectronics]

I am no expert, but I have been involved in the painting of enough marine structures to know the basics.

Of course, but the very first principle of painting is that you start with a clean substrate.

 

[Dcal]

Which is why you clean before you blast.
The blasting process should not add anything nasty.

That hardest thing to get off previously rusty steel is ingrained salts and they will be worst at the bottom of any pits.
The only way to get rid of the salts is cut out the contaminated steel, or to dissolve it and fresh water is the cheapest solvent.
Panel wipe or thinners won't touch it and most people don't test for it so it's normally just left, but it's the biggest cause of the early breakdown of coatings.
Water makes a mess of the nicely blasted steel so you need to get rid of it and blast again.

There are products like Chlor-X that are supposed to be better than water, but water works well enough and if the salt levels are low enough a good coating, like epoxy primer, should give a satisfactory service life for the coating.

 

[bricol]

Afraid water is going nowhere near my freshly blasted surfaces.
I will wipe/spray over with thinners, blow it off or wait for it evaporate, and then apply epoxy primer.

Still stuck under a daily use car 18 yrs later, so something worked reasonably well.

Paintwork people will see, I pay someone who knows what they are doing. We'll pretend we can't see the micro-blisters all over the roof . . . I need to find someone who knows more about what they are doing . . .

 

[ajlelectronics]

I will wipe/spray over with thinners, blow it off or wait for it evaporate, and then apply epoxy primer.

Use brake cleaner, wet enough so it will drip off before flashing.

 

[Ross365]

The blasting process should not add anything nasty.

You can get particles of the blasting media firmly embedded in the metal and then the paint does not like that. It shows up most on a surface that was subsequently been sprayed matt black. Going over a blasted surface with a wire brush (to get out any embedded bits) is sometimes worthwhile.

 

[MetalMonkey]

I can't speak for industrial blast cleaning, but I've done quite a bit of restoration.

I typically use aluminium oxide grit which leaves the surface covered in a fine black dust. It doesn't blow off because it clings to the surface via static.

If blasting in a cabinet the media is recycled along with all the crud that comes off, which is blasted back on to the clean steel, so there is always some contamination.

On a course blasted surface it is difficult to wipe it down with shop towels, but you can use linen rags, hose on the panel wipe and get passable results. Once dry you can blow off the lint or tack rag as required.

I can see it wouldn't be viable on big structures like a bridge, but then I don't paint bridges...

I've sent bigger stuff out for blasting, and made the mistake once of taking up their offer to prime them, which I assume wasn't pre cleaned and was already flaking off by the time they reached me - took me a while to strip that all back and redo it.

On the other hand I've sprayed the pot dregs on garden furniture with nothing more than a quick brush down to get rid of the cobwebs and it survived for years.

So yes you can paint over muck, but panel wipe is cheap and paint, blasting and time isn't. So I prefer to get it as clean as possible before laying anything on, to give it the maximum possible chance of success.

 

[brightspark]

i dont use copper slag on decent gear that leaves a bit sooty i use fine sand and crushed glass and after the 1st coat of acid etch and primer it gets a fine sanding then you can use pannel wipe then your top coats as for using the media twice i dont and never have done you end up hurtling various size media round thats filthy and difucult to regulate and wastes time and all you do id hurtle a load of dust around some people are happy using it others take it to blasters who just lace it with coarse grit at about 300 cfm and ruin stuff