K&N make a brush on rust converter. You can find it a all motor part stores Repco etc. It is in a little plastic jar. Creamy brown in colour. POR 15 also make a great rust proof paint. I had my mini beadblasted and then i painted the shell in it. It is rock hard, hammer tough and where i have grinded or cut metal is the only place it has surface rust on the whole shell after 2 years. The only problem is cost. $250 for 4 litres plus you need a special primer to pain over it.