Can You Plaster Over Hardboard?
Hardboard is commonly used around the home as a backer board. Because of this many people, you included, have wondered whether you plaster onto it. Well, let’s have a look and find out.
No, you should not plaster over hardboard. Hardboard is just too flexible of a surface to plaster over. The movement in the hardboard will result in lots of cracks in the plaster.
Beyond just the movement factor there is the issue of the hardboard soaking the moisture up from the plaster. This will cause tons of adhesion issues down the line which will probably result in the plaster simply falling off the hardboard.
What Should You Do Then?
If possible replace the hardboard with plasterboard. this will solve all of the issues and give you the perfect surface to plaster to.
If you cant remove the hardboard then can you just add some plasterboard on top?
If you can get it secured strongly enough that there is no movement and it still sits below the level of your wall then this will be able to be plastered on top of.
What If I Have No Choice?
If you really must plaster onto hardboard then it is important to make the conditions as good as you can. I would try and use some sort of waterproofing sealer on the hardboard first.
This will give a really poor surface for the hardboard to adhere to, so now we need to fix that.
I would then ass some blue grit on top of the sealer.
Helps Adhesion on non-porous surfaces like floor tiles, porcelain tiles, terrazzo, quarry tiles and ceramic tiles
The blue grit gives the perfect surface for the plaster to adhere to.
So by doing these two steps we have hopefully taken away the moisture issues whilst also giving ourselves a really good surface for the plaster to adhere to.
This obviously does not address the flexibility issue though, which is why even with all of these additional steps I would still advise you don’t plaster onto harboard.
What Is Hardboard?
Hardboard is the harder brother to MDF. MDF stands for medium-density fibreboard, hardboard is high-density fibreboard.
It is stronger than MDF and much denser. The fibres used in hardboard are exploded fibres which are then highly compressed. This is what allows it to reach such high density and strength.
Hardboard is often used as pegboard, or in furniture construction, particularly as a backer board.
You don’t see hardboard in thick sheets, you only ever see it in the really thin sheets.
I’m guessing this is because a thick sheet would be incredibly heavy and probably stupidly expensive as well.
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