Hello Monarch,
For boiler lagging sealers i would go down the oil route, using Starbrite linseed oil applied by cotton cloth to the outside only and give it about 4-5 coats allowing it to dry between coats.
Any good boat chandler will sell you a small bottle for about £8.00.
Don't forget to let it dry completely (a few days should be ok) before firing the boiler as you could get some combustion/charing on the outside of the boiler.
Hope this helps
Ian