Welding is ideal but
Often not possible, for whatever reason.
Here's how we tackle the problem (nearly always one particular brand in the 'States, GE Filterflo tubs, but generally applicable):
1) Clean, clean, clean the entire tub inside and out. Not only to find damage, pinholes, etc., but because any repair is better when it's not contaminated by moisture, grease, rust, dirt, etc. Silicone is your enemy.
2) Remove rusted areas all the way back to healthy metal. You may well find a trace radiating out from the damaged area which, at first, looked healthy but would have quickly failed.
3) Composites hold better than single materials. If the areas to be repaired with JB-Weld (or equivalent) are large (bigger than 2cm diameter), make it a several step repair with some fibreglass from the body shop anchored across the hole. Don't be afraid to think in three dimensions - you can built up the plug from the bottom, too.
4) Once everything is finished, one of the waterproofing PUR products (or equivalent) over the entire surface will go a long ways toward making things last.
5) Use it. Use it lots. I don't care what anybody says, appliances were made to be used. We have a saying in German: Wer rasted, der rosted. Goes for machines just as well as people.
