Bottom up approach
One factor tells more than any other. "Preparation," Dave Herrod of ABP admits, "is always a problem." He adds: "Virtually all paint manufacturers specify that the steel should be shot blasted to a certain standard."
To answer this, water cleaning is now gaining favour on site - a technology that uses fresh water or a water/abrasive mix, (e.g. almandite garnet) at very high pressure. The pressure for water blasting can be 60,000 psi (4,000 bar) but a clean up with air, water and grit - slurry blasting - can be effective at the lower pressure of 130 psi (9 bar) depending on what needs to be done.
However, even though with adequate pressures you can get back to bare metal (according to Swedish surface preparation standards, Sa 2.5 or 3), one big disadvantage with the water cleaning method is that unlike grit blasting, it leaves much less of a surface 'profile' - you get back what was manufactured in the first place, though you can get a bit of a key for paints if you use an abrasive mix.
But sometimes the previously painted surfaces cannot be completely taken back down to 'bare' (the Sa 2.5 or 3 standard) so there is an issue with how much energy you put into taking off layers, to try to get to a stable substrate that can be adequately coated - will a paint be happy sticking to a wire brushed surface, the Swedish St2 standard?
There are, of course, the surface tolerant coatings, most based around aluminium-pigmented mastic epoxies. You can prepare with wire brushes, and even use a rust converter, but you still don't get the holding time. As Mr Herrod notes: "Some paints are better than others at accommodating imperfect preparation but it really comes down to accepting that recoating will be needed more frequently."
