Pinhole leak in the recovery tank.Help!!!

Hi,
I have Dodge Ram Van with a 8 year old  Hydramaster CDS 3.8 in it.I bought it used from DA Burns in Seattle.
About 1 1/2 years ago the recovery tank developed a pinhole leak on the bottom of the tank. It's located on the passenger side about 2 inches from the front edge of the tank. I patched it from the bottom  with  a  product called Aluminox Epoxy Stick. That's two component putty made for marine applications. It held up until yesterday when the patch pretty much just fell of. The waste water must have gotten behind the patch and loosened the bond to the tank.
So I was going to patch it with the putty again but  my girlfriend had what I think is a much better idea, which I want to run by you guys.
So here it is.
She suggested that I drill the pinhole out large enough to get a bolt through. I can actually reach the hole with my powerdrill (I have long arms) Then I could put two metal washers on either side of the tank and in between that, to create the seal I would sandwich couple of soft washers. I want to make the washers as large as possible because the surrounding area might be weak as well and that would give a larger area more strength.
So my question is: Could I use regular rubber washers for that or will they disintegrate from the waste water and the chemicals?
Is there a better gasket material that I could use as washers?
By the way I completely cleaned out the tank and checked it with a shop light to see how the rest of the bottom is holding up and it seems OK . Even if it does the same thing somewhere else, as long as I can reach it I could do it again.
The machine and the truck are still going strong but I don't have the money for a new tank and welding it without taking it out of the truck is impossible.
But this fix really seems like it would be my best bet.
What do you guys think?
Do you have any other suggestions how I could make the tank last a few more years?