I have a 1950 D4 Caterpillar. It sat outside for a couple years. It has BAD rust in Fuel Tank. Dosen;t take long to plug fuel Filters. The tank is the seat tank. How can I get this cleaned?  I am also thinking about installing a in-line-filter, if possible. Can you give me any suggestions for cleaning Thank you/

you could have it steam blast cleaned. if you know some one a steam boiler get the to steam blast it for you.

the other way is get some caustic soda based degreaser mixed with hot water and some light chain or a couple handfuls of small nuts strap it to the wheel of a tractor and go for a drive round the paddock a few times. the idea is the chain or nuts knock all the scale loose so it stops comin off and pluging the filters.

then treat it with a fuel tank sealer such as redkote or the por15 product or similar. ive used the KBS products one and it was easy to use.

as for a inline filter have a look at the parker racor range or the delphi 796 filter/water trap you would just change from the rigid copper tube to fuel hose instead

I had a rust problem, having more old tractors than brains I used a different tank finally. I was able to find in line fuel filters at NAPA in larger line sizes. On the D6 I think 1/2 inch line. In the end much cheaper than buying the woven string filters. Using a filter base and spin on filter is also very doable if that is how you want to go.