I've used these before for master cylinders. Worth giving them a ring first to discuss.
Impregnation Services Ltd
'Porosity Sealing Solutions'
Unit 1 Cowhill Industrial Estate, Cowhill Lane,
Ashton Under Lyne, Lancashire, OL6 6HH
Tel : +44 (0)161 344 1004
Fax: +44 (0)161 344 2428
http://www.impregnation.co.uk/