You should definitely check for leaks first. Have a look under the carpet below the heater and check the matrix under the bonnet. You could also try and give it a compression test too if you can to see if it's holding and equal pressure.
Never used the sealing stuff before but if it has blown - even slightly- then it usually means it'll just get worse no matter now much seal you put in so I'ld just slap a Jew one on