Gary
I have cleaned a dozen or so tanks.
I start with pressure washing. If stubborn build up is seen (I use a inspection flex cam to check) then I will run the pressure washer with some simple green or purple power to blast out the caked on.
Home Depot, lowes, or your local pool supply sells muratic acid.
I plug all the holes and fill the tank half full of water then pour in the gallon of muratic acid and top off and seal the tank.
Let sit a day or so then dump and flush with running water till all runs clean.
If needed repeat the steps
When I can see nice clean bare metal then let dry (here in AZ that is sun bake for a couple hours, but in wet or cold climates a hair dryer or heat gun will do nicely). Then last use the good old POR red tank sealer.
Watch your pickup tube if it has a screen remove before cleaning, and especially before adding sealer. I also run a old piece of outer speedometer jacket or some flex wire through vent and pickup tubes after a few minutes of the sealer drying to make sure lines did not get blocked. You can also use low pressure compressed air to make sure they are clear.
I should mention it is wise to check these for full or partial blockage prior to cleaning if they are use some solvent like carb cleaner or worst case a the drill and a piece of speedo cable to ream them clean.
