Check fuel tank ,before removing.

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We have built many fuel tanks at work over the last 40 years. All threaded fittings are plugged with caps or plugs, and hose connections are blocked with short lengths of hose with steel, plastic or wood plugs. Minimum of 3 psi, max 5 psi is put on the tank and the fill closed, leaving the tank for 8 hours to see if any pressure drop(indicates leaks) If none you are good. Most holes will usually be
pinholes in the bottom of the tank. Could immerse tank in water to check for leaks, but the tank will be buoyant by 6 to7 pounds per gallon. Hard to hold it under water.