Vintage Tiger Tanks Anyone ?

I'm going to seal the inside of my 94 Tiger tank with https://cwylde.co.uk/flowliner-tank-coatings/ unless anyone
tells me it is a bad idea. Question. How long should the tank be left to dry out before the coating is applied. Also, should the tank be washed out...with water, with soap and water ? It is very clean inside and because it is plastic there is no rust. Instructions say remove everything from the tank and seal...I did but......the overflow drain plug on the underside of the tank snapped off when I tried to remove it. Turns out I didn't need to remove it because there is a dedicated overflow pipe inside the tank from the filler cap. If only I had know I would have left the bloody thing well along. Tried to get the rest of the drain plug out with a screw puller but all that came out was some copper pipe. Tip. Never remove the drain plug for any reason. It is aluminium and will no doubt have corroded so will almost certainly snap off.