Rust converter, seam sealer - pre and post welding process?

[norsa]

Quite stiff, coarse nylon bristles. http://www.ebay.co.uk/itm/3M-Sealer-Brush-For-Brushing-Seam-Sealer-Stiff-Bristled-/280870239070

 

[Daz1968]

For inside sections I have used ferrozinc water based rust converter this can be sprayed inside box sections like waxoyl, then once dry spray waxoyl in as well to be sure, it's the stuff they use on steel boat hulls so is pretty good. It can also be thinned with water to aid spraying.