I am getting a custom stretcher made out of MDF. What should I use to coat the MDF to make it archival?

I would use several thin coats of Gesso, but be aware that you may have problems with the MDF splitting when you staple the canvas so don't staple close to the corners.

Thanks once again, Randy! Happy New Year!

Eleanor,

I agree with Randy. Gesso would be my choice or suggestion but I am questioning the mdf. Not my first choice for stretcher bars. Be careful stapling into it and most adhesives don't like to stick to it either. Wood glue is the only consistent thing I have found. Great for the wood, not good for the canvas. I am curious as to your reason for mdf. I typically use poplar (my first choice) kiln dried to 10 percent or kiln dried fir or pine. Good luck and let us know how it comes out.

I have to agree that using the MDF is questionable and I am using it for round stretchers. This is an experiment and I haven't received them yet. I'll let you know how it all works.

MDF does not have any lateral stability at narrow widths so the members of your frame will need to be much wider than typical wood frames. This is because MDF has no "grain" which is what gives wood strength in one direction. A good quality cabinet grade plywood would be infinitely more rigid since it uses layers of real wood, but alternating the grain to increase stength where MDF is simply fine wood shavings glued together, and while dense and hard on the surface, as well as very stable in terms of movement, it breaks as easy as a saltine cracker

I agree plywood would be a better choice, Alternating the grain does not actually increase strength of plywood compared to the equivalent cross section in the timber plywood is made from, rather it prevents it from warping and makes it a stable material dimensionally. Which is exactly what you want for a stretcher and is much much stronger than MDF. You would have seal MDF well as it absorbs moisture readily and also uses formaldehyde in the glues which can cause staining and would be detrimental to archival media.

Not to argue the point but alternating the grain actually does GREATLY increase the strength of plywood. The reason is the weak points (grain) are reinforced by alternating the grain which makes a much stronger surface than a regular piece of wood and much less susceptible for cracking and breaking - for instance when used as a subfloor - plywood is infinitely stronger than boards of normal wood. I was in the building industry for over 25 years.

I should have been clearer, plywood has about the same mechanical properties in all axes, while wood varies in properties depending on grain direction. Wood is strongest along the grain and weakest across the grain. This link from the wood handbook explains: http://www.woodweb.com/Resources/wood_eng_handbook/Ch10.pdf go to page 10-11, last paragraph. The link also explains the various grades of plywood, cheap grades are full of voids and would be unsuitable, marine ply is water resistant and has no voids.

I agree you would need large cross sections in mdf you can break small sections like a 2x1 in your hands in MDF, no chance of doing that with timber or plywood.

Biggest drawback to plywood for stretcher bars would be weight, cost and splintering. We have now made several thousand stretcher bars for our wholesale canvas business and hands down the best material is kiln dried poplar. It is less likely to warp than pine or fir and is actually a little lighter. I have used frames made of HDF (high density fiber board instead of medium density fiber board, MDF) and they are stable, straight, reasonably strong but don't take a screw well and you have to be sure to get a solid screw set in it.

I am curious to hear Eleanor's reaction to the wrap when she gets it, maybe it will something to experiment with.