laying hardibacker on plywood sub floor

All the instructions I've read say to install hardibacker on "exterior grade plywood". I didn't use exterior grade plywood, but I do have a solid 3/4" plywood sub floor. Do I need to pull up my sub floor? Should I prime with exterior grade primer and paint it instead? Do I really not need to worry? Does the moisture in the thinset soak into the plywood? I want to do it properly but I really don't want to pull up my plywood.
Ian
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<< didn't use exterior grade plywood, but I do have a solid 3/4" plywood sub floor >>
Good substrate for any flooring.
<< Does the moisture in the thinset soak into the plywood? >>
Where else can it go?
<< Should I prime with exterior grade primer and paint it instead ?>>
No, too much work. Instead, open all the windows and slop on a heavy coat of the cheapest urethane floor varnish you can find. When the fumes dissipate and the stuff is cured hard, close the windows and touch sand the lumps off and proceed with the hardibacker installation. This probably is overkill, as most folks would just do the hardibacker without worrying about the plywood. Plywood makers these days seem to be using exterior grade adhesives across the board. I haven't seen any strictly interior grade plywood for a long time. Unless there are telltale green stripes on your plywood edges, it may not be interior grade. Good luck on your project.
Joe
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