It appears there are conflicting recommendations on various manufacturers’ websites regarding sizing HDF/MDF before applying acrylic ground. Looking at the table in the “Adhesives and Sizes” document, three coats of acrylic ground should be enough to protect from any noticeable migration of mediums or solvents through the support, so shouldn’t it also be sufficient to protect the work from the chemicals that might migrate from the support?
Assuming that sizing is still recommended, should PVA glue with a pH of 7 suffice? If so, should it be diluted? How much?
Also, when using HDF/MDF that’s laminated on one side, is it fine to leave the laminate as is, since it’s already a protective layer?
Thank you for the very informative answer. Just one more thing – should I sand the fiberboards after applying the sizing to increase the adhesion of the ground?
Thank you. Should I assume that the boards I’ve primed (with acrylic ground) without sizing them first are not good for painting? Also, if I over-sand them and have the fibers sticking up, should I assume they are not good either, regardless of future sizing and priming?
Alright, thanks for all the explanations.
A good quality PVA (with a pH of 7 as you suggest) would be fine as a size before applying an acrylic ground. In fact there is often more concern regarding support induced discoloration (SID) which involves the leaching and migration of materials FROM the support up into the ground and/or paint layers rather than the other way around. Again applying a layer of size such as a PVA size or an acrylic dispersion medium/gel before applying a ground will certainly help to avoid this issue. You can dilute the PVA size to a brushable consistency…it is better to apply a coat that might be a touch dilute rather than applying one that is too thick. You can always over dilute the PVA size and even apply 2-3 coats as well. As for applying an acrylic ground, yes, 2-3 layers should be fine…if you have a size layer it likely fine to even apply 1-2 coats as the size layer serves as an additional layer that is preventing SID from occurring. Finally, yes it is fine to leave the laminate on…when painting on HDF/MDF it is always a good idea to coat the back with a size layer and even a coat of paint to help prevent directional warping from occurring so the laminate will most certainly serve that purpose.
This is covered a bit in our “Rigid Supports” document that is located in the resources section so I will copy and paste the relevant information below. You do not really want to sand after you size….it is better to do this beforehand.
Gently sand the face of the panel with fine sandpaper (e.g. 220 grit) to provide a slight mechanical tooth but be careful to not overly roughen the surface and unevenly expose the wood fibers. This can lead to irregular swelling of the substrate (particularly when water-based sizes, sealants and/or priming/ground are applied).
The panels you have already prepped are likely fine….just record that on that back of your painting should any unforeseen issues arise (e.g. note that no sizing was used only priming….). As for the ones you have “over” sanded, you will simply need to add more layers of sizing and priming than you normally would in order to evenly coat everything (again making a note of things on the reverse).
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