Mix it up thick with WPP powder, apply with a brush or roller - it clings to your work and washes off with a hose. It doesn't store well - I guess the acid works on the paste too. I use it quite a lot, on gel coat, flow coat and tiles. It works a treat. You can use it on stainless steel too.
All the usual precautions apply as regards personal protection. You might want to keep it away from galvanised steel (hint: trailer) - it comes up very clean but much thinner!!
Start with 10% Acid to 90% water/paste