Adhering a mirror to a wood panel with mirror mastic: what primer would you use?

[Ken Cobbing]

Hi all,

I'm finishing up an overdue medicine cabinet project, overthinking every step of the way, I'm sure. The cabinets and doors are finished and all I've left to do is attach the mirrors to the wood doors.

The design is modern with no mirror frame, and the mirror/door overhangs the cabinet on all sides. The mirror sits on top of the door and the profile of both is identical with large corner radii. I've attached a photo of the doors I've made, as well as the style that SWMBO asked me to use as inspiration. In short, the mirror needs to be glued directly to the front of the wood door.

The mirror mastic manufacturer (CR Laurence) states that all porous surfaces (including wood) should be primed before using the mastic. They sell an acrylic primer in gallon cans for over $100.

I'd like to use an alternate primer since I don't need a gallon and the price is a bit ridiculous. What primer would you suggest? I'm a bit uncertain about how exactly the primer helps adhesion between wood and the mirror; is the purpose simply to seal the porous wood so that the mastic does not have moisture pulled out of it?

My first instinct is to use Zinsser BIN shellac primer since this would do a great job of sealing the porous wood by soaking into the wood fibers. On the other hand, the manufacturer uses an acrylic primer, so perhaps Bullseye 1-2-3 would be a better choice?

Any thoughts on the necessity of primer and/or the best choice for the intended purpose?

Thanks for reading :)

[Alex Zeller]

I'm not sure why you would need to do anything to the wood unless they are worried about staining the backside of the wood. Mirror mastic is very similar to construction adhesive (like PL400). It'll stick to almost anything. I've pulled mirrors off of painted sheet rock walls and the paper will tear off of the rock before the glue lets go. If I had to guess MM is designed not to compromise the backing on the mirror and possibly stay slightly flexible. I'm sure any primer you put on the wood will do fine. What I would be a little worried about is wood movement. Being in a bathroom humidity changes are going to be more pronounced than anywhere else in the house. I'm not sure if the MM will allow the movement needed. Maybe someone else has better insight into it.

[Jim Becker]

I agree with Alex...I don't think I'd use anything other than preparing the wood surface as normal.

[Ken Cobbing]

I thought I'd give an update for anyone interested. I did decide to prime the wood panels (doors) and I used Zinsser BIN alcohol/shellac primer. I figured that maybe it is not necessary, but the manufacturer recommends a primer, so it couldn't hurt.

At the same time that I used the mastic on the primed door panels, I also used some mastic to stick two unprimed 2x4 scraps together.

The cure time is stated as a month, and it has almost been that long.

Today I tried prying the unprimed 2x4 scraps apart. It definitely took some effort, and the attached photo shows an interesting result. On part of the scraps you can see that the wood itself tore out in a large chunk. However, next to that tearout, you can see that the mastic did not really adhere to the wood and came apart much more easily.

I suspect that if I had primed the boards that the entire mastic blob would have tore out wood when prying the boards apart, and I think that following the manufacturer's advice to prime porous surfaces is a good idea. Hopefully my choice of primer was a good one.

[Alex Zeller]

As you said, I think you are over thinking this. There's not much force on the mirror trying to pull it off of the door. It's not like gluing a woodworking joint. The key is to glue it in place and hold it there until the mastic sets up (which doesn't take very long. Once that happens you actually want the mastic to not stick too well just in case the mirror breaks and needs to be replaced. I've removed a few and usually it requires taking shimming shingles between the mirror and wall to apply a steady amount of force over a large area. If lucky the mastic will stretch some and you can get a blade of some sort in the gap and cut the mastic. Otherwise either breaking the mirror or damaging the wall is the most likely outcome.