Alby, As long as the Humbrol is fully cured, I don't think you'll have a problem using the Model Master Metalizer Sealer as a glosscoat - other than the strong fumes; but if you've used any of the MM Metalizers, you're already aware of that-
The nice thing is that the sealer sprays so thin, and being lacquer based, it dries very quickly...and certainly will be an excellent barrier for an acrylic wash.
Good luck: Knowing your work, I'm sure the results will be spectacular!