Wanted - Tips on working with a large board

[Richard]

I am planning to produce a number of icons on MDF panels, all larger than A3, and intend to gesso all surfaces. Generally I give my panels between eight and twelve coats of gesso and tend to add a new coats of gesso as soon as the previous coat has lost all surface moisture and is a dull grey colour. Until now I have only gessoded front and edges of panels and would appreciate advice on how other painters handle large panels when gessoing all surfaces with out marking or damaging the surface.
Best wishes to all.

[Alessandra Kelley]

I let my gesso dry between coats. The first few coats I can put on in one session; after that it's one coat and give it a few hours. Naturally, this works best when priming several panels at once.

I understand that quite a few people put on their gesso while the previous layer is still somewhat damp. It seems to work well for them, but time constraints for me necessitated the dry technique.

As far as I know, although there are hypotheses about the superiority of one method or another, there is no clear evidence that either is better. I have had no adhesion or cracking problems with my gesso (and I like being able to sand it smooth in the middle of the process).

Of course if you let the gesso dry, there is no difficulty in gessoing fronts ad backs of panels.

[PhilS]

Richard,
My method (for the past 15+ years) has been to apply 6-8 coats on the back side, allowing 15 to 20 minutes drying time between coats. Then I flip the panel over and do the same on the front. Very simple. I smooth the finish side with a wooden block dipped in water because this doesn't remove any gesso, just smooths it. I do usually finish off with a light sanding using 220 grit sandpaper, but that removes very little.
For a large panel, I would advise cradling. I generally don't go any larger than 24" in width or height and haven't had any warping problems with panels that size. My framing technique also acts as a stabilizer.
There were some discussions on MDF a couple of years ago. You might want to check out the archives and see if they are still there. I seem to remember some potential problems with MDF...
Phil

[jeff]

I have used quite thin MDF for sizes as large as 1x0.9m without any warping so long as I gesso both sides of the board. I usually only put on about 3 coats, but fairly thickly and rely on sanding back just before painting. The wooden block sounds a good idea.

Jeff

[RobM]

Richard,
I like to paint on a large scale relative to ET and have recently been experimenting with Ikea table tops!! They are constructed with a perimeter frame (about 25mm), with a 'honeycomb' of card sandwiched between a thin MDF backing board and a plastic laminated top and sides. The MDF is pre-drilled but I just fill them with any old filler. I usually get the white table tops, sand the plastic laminate of the 'top' surface then apply a couple of coats of gesso. I then size and gesso the 'bottom' MDF and bring that up to the finished surface. So far the experiments have worked well, absolutely no warping. The beauty of these table tops is that they are ready constructed, very cheap and do the job admirably.
You just have to accept the sizes that they provide though. I use the white 1000 x 600 mm.
Rob

[Richard]

Thanks for all the advice I will try and steer a course using a thinner MDF than I usually would and allowing layers to dry more than I usually do. Unfortunately we do not have an Ikea in Ireland yet! It will be some time before the icons are needed so I think I will try out a test panel full size and see how it settles over the next six months.
Regards and thanks Richard

[Bert Congdon]

I coat them rapidly as you do. I glue cotton canvas or linen on thin plywood, and I cradle with 1x2 poplar. When dry, I shellac the back. They are large...24x30 inches. Bear in mind that I am no expert. I do not paint that much since having a stroke, but I have some panels that have been around a few years with no problems. Also, shellac has a short shelf life, two years at the most, and how long has it been in the paint store. Go beyond its shelf life and it won't dry. I keep the dry shellac flakes and mix fresh with alcohol each time I use it. Bert

[Richard]

Thanks Bert, very useful information, would you please add some more information on how you make and attach your "cradle". Having tried to use some store bought shellac whici had gone over its life span I changed over to making my own, a very easy procedure.
Richard

[Bert Congdon]

Sure thing. I measure the panel, cut the one-by two's to length, and miter the ends. Using yellow carpenter's glue and clamps, I glue these to the perimeter of the back, one piece at a time. When set, I drill a hole through the miter joint, glue up the proper size dowel and drive it through the miter joint. This is just an added precaution. A nail or a screw won't do as I am sure you know if you have come across an old frame that has a broken joint. The frame is useless until reglued, and the nail that might still be hanging it together did nothing to strengthen that joint.

On a large piece, I will probably glue a piece across the middle of the back. I will drill a hole through the perimeter piece and into the end of the cross piece, glue up a dowel and drive it in. After this I will glue on my linen with carpenter strength hide glue. Yes this is the same glue that I use for the gesso, but stronger. I have found no difference in hide (from cows) glue and rabbit skin glue. The company I buy hide glue from also says there is no difference if the numbers are the same.

Carpenter strength is made in this manner: put you glue in a container, add just enough water to cover the dry glue and gently warm in a larger container container of warm water. I no longer use a double boiler as this can be too hot.

Ask me what time it is, and I tell you how to make a watch. Bert :lol:

[DLH]

You can tell exactly how long shellac has been sitting on the hardware store shelf. The date of manufacture is printed on the bottom of the can, but when you look lift the can gently and don’t tilt it. The best shellac has little or no wax. Wax is inherent to raw shellac and it is what makes it look cloudy in the can. As it sits the wax settles to the bottom of the can leaving absolutely clear material on top. It takes months for this to occur.

Zinsser is the only brand of canned shellac that I have ever seen. They have a proprietary treatment they claim extends shelf life to over five years. While I can’t speak to that, I often use material more than three years old. I have never had it fail to dry perfectly.