in the case where ready primed mdf feels rough on the moulded sections is it best to sand them prior to applying the gardz but by doing so would remove the factory primer?
Only sand if you can do it without exposing the raw MDF. It may be more beneficial to use primer instead of Gardz in your scenario. Sounds like you need to build up the prime coat to an acceptable surface before applying paint.
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I'm going to try brushing out some unfinished trim with gardz. I imagine that sanding it smooth would be the first step, application, then nail hole fill.
I saw a post somewhere on here about a water-based product that recommend wetting the surface before applying the product. It wasn't one I was familiar with so I don't recall the name. I just keep thinking about it... did the water decrease drag it help the paint absorb into the prime coat better?
Pre-sanding trim would be specific to the type of finish on the MDF. Some MDF has a very thin factory primer that really shouldn't be touched. It can cause more harm than good by sanding it. If it burns through too easily, best not touch it.
I never tried pre-wetting a surface prior to applying paint but I am left with no valid reason to explore it.