DON’T put any sealant into the oil compartment !!! Put in the petrol section only. It is so viscous that it won’t go through ‘pinhole’ size holes in the solder between the two compartments. In recent years I have had endless problems with sealing and unsealing tanks, Scott, MZ, and Jawa in the past year alone. I suggest you go on line to “Tank Care Products” who are in Norwich. The guy is called Potter, and he can help you.
The very important thing is absolutely perfect cleaning out of the tank to get rid of ALL petrol and oil residues, plus any flaky rust. You will have to do this to both sections of the tank, but the sealant ONLY in the petrol compartment. It is most important that you follow the instructions to the letter ! When using the various cleaners and sealant you have to be very careful to wrap the outside of the tank in at least two layers of thick polythene to protect the paintwork, sealing around the filler hole with duct tape. The holes for the petrol tap and drain cock have to be plugged carefully, using PTFE tape on the threads, so that the plugs can be removed later, and to keep sealant out of the threads.
Since they started putting Ethanol in petrol there have been thousands of problems with it softening older types of sealant, such as the original ‘Petseal’, but newer products claim to be ‘Ethanol Proof’, or the slightly worrying ‘Ethanol Resistant’. Anyhow, good luck, and do phone Mr Potter !