sealing hardboard with shellac

Honestly, I have not experimented with shellac to an extensive degree. It yellows and checks if used beyond the thinnest coating, especially if exposed to the action of air and light. Some painters in tempera have used water white shellac in extremely thin coats with no obvious ill results, but tests on the macro scale suggests that it is a poor “surface” coating. It would probably be fine for your purpose. Probably the 33% is a good place to start and after drying check to make sure that there is not too substantial a layer or, conversely, if it needs another application. If the first, sand it back and thin with more alcohol, if the second, apply an additional layer. Others may have more specific recipes for you.
Having written this, I would suggest a different material for the purpose you mention. Paraloid B-72 dissolved in xylene (applied in outdoors or in a fume hood, etc) would do the same thing that you desire (a water free sizing/sealer)­­­­­, be composed of a material known to be very stable, and would be more sympathetic to and accepting of your subsequent application of water born acrylic dispersion medium. A Google search should provide you with an online source of this acrylic resin.  

​Oddly enough we received a question yesterday about using clear shellac as a sealer against moisture, so given its relevance let me share the reply here as well:
“Shellac is often used by woodworkers as a preferred sealer for wood as it is not waterbased and so will not raise the grain, dries very quickly. However we would opt for a pigmented shellac – such as BIN’s Original – as being a better water moisture barrier and take that information from the Forrest Products laboratory, which are truly the experts when it comes to research into wood finishing and protection. You can find information about their recommendations in various places. Here is one link to a list that includes pigmented shellac, which rates surprisingly not all that far behind even a two part epoxy:

https://www.fpl.fs.fed.us/documnts/techline/protecting-wood-from-humidity.pdf
Another document supporting pigmented shellac over clear can be found here – a much more technical document to be sure but will quote the relevant part from page 14:
https://permanent.access.gpo.gov/websites/www.fpl.fs.fed.us/documnts/fplrp/fplrp462.pdf  -(Note: MEE stands for Moisture Excluding Effectiveness. The higher the number the higher the percentage of blocking moisture.)

‘The low values of MEE14 for latex finishes stand in contrast to those of the shellac-, varnish-, or paint-based finishes that we evaluated. A white shellac (alcohol solvent) (finish 23) with an MEE14 of 73 percent for six coats was less effective than a pigmented flat shellac (also alcohol solvent) (finish 60) which had MEE14 of 83 percent. For each coat applied the MEE increase was greater for the white shellac than for the pigmented shellac. This greater increase in MEE with each successive finish coat for a nonpigmented versus pigmented finish was also observed with the gloss urethane varnish (finish 13) and the aluminum flake-pigmented varnish (finish 43). Increases in MEE for the paints (finishes 67 and 77) were similar to those for the pigmented varnish and shellac. Browne (4) has done an extensive study on the variations of MEE for a linseed oil paint according to the nature of the pigment. In general, pigmented finishes have much higher MEE than unpigmented finishes for any specific resin system.’
As a last entry into the Forrest Products Laboratory Literature on this – and a bit more accessibly written – take a look at page 16-14 (Chapter 16 page 14) of their Wood handbook:

https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pd f”
In the end we agree with Brian that B72 would likely be a more durable product and should lessen moisture sensitivity, although am not sure that it would be more effective that pigmented shellac. That is just not something I have seen compared.

​Sarah. Thanks again for your fact filled response.

​In terms of amount of coats I would follow whatever the directions state. I know when I work with BIN Original, which is likely the brand of pigmented shellac primer you saw, I apply two coats simply because I always feel more confident that everything has been covered and that the film is more even throughout. In truth two coats will still appear quite thin, so no fear that you are building up an appreciably thick layer. And with shellac, because it does grow more brittle with age, keeping things thin makes sense as well. Finally, would not thin it further – BIN comes quite thin and ready to use already, just make sure to stir it up thoroughly.

To be honest I doubt that you would need to do any sanding – the shellac pentrates easily – but if you did it would be the lightest of touches.. However, I would likely wipe down the surfaces with alchohol first just a a form of degreasing.