Secrets of Gluing with Weldwood Contact Cement
Prep Work
Read the directions on the can.
Several days before the gluing is to be done, put the glue in a warm room and start inverting the can every 6-12 hours. You want to get the hard-settling off the bottom and start getting the glue properly mixed.
It is very hard to get the settling mixed in when the can is full, using a wooden stirrer. Save an old can and pour the glue back and forth to mix it.
Tools Needed
Find a cheap 3" throwaway brush that you can use to brush out the glue. We try to conserve on brushes by saving them inside a ziploc bag when finished. This keeps them usable for about a week. You could try putting the brush/bag in the freezer - they might keep longer there.
(A stiff, glue-dried brush will often come back to life when used. However, because of the stiff bristles, there is more likelihood of scraping off the primer if it has not fully cured.)
Locate some silicone release paper, at least as large as your gluing project, preferably larger. United Manufacturers is my source
Acetone and clean rags, for clean up of spills. Also use Acetone to clean the metal surface prior to applying primer.
Scratchy pad and Dawn detergent to clean the back of the metal. Rinse, then fast-dry with acetone soaked cloth.
2x4s to raise the gluing surface off the table
Automotive oil-based primer - I use Rustoleum 2081, from Home Depot or Lowe's
Varathane or similar, for coating the wood prior to gluing. Don't use a solvent- or oil-based product - tShe solvents in the contact cement will dissolve them.
Copper
If the copper was flame-patinated or is warpy in any way, it will need to be flattened and planished as much as possible before gluing. Use a rubber dead-blow hammer and put the copper on wood. Don't use metal on either side of the copper - that will work harden the copper too much.
The glue does not bond directly to copper - it will fail after a week or so. That's why we use primer.
The back of the copper sheet must be wire-brushed by hand, cleaned, then primed. Don't use a grinder or drill with a wire brush attachment - that would cause the copper to get work hardened.
An oil-based auto body primer is preferred – this needs to be dried in an oven (150 degrees for 30 minutes) or air-dried for 3-4 days at room temperature. I use Rustoleum's 2081.
Other ways to speed curing of primer - put it outside in sunlight, if its warm enough. Put the primed surface in front of an electric or LPG heater.
Aluminum
Planish and hammer smooth if needed. You may need to heat the aluminum to anneal it. Take an oxy-acetylene torch, light it, then turn off the oxygen. Evenly apply the soot from the burning acetylene to the aluminum, then turn on the oxygen and burn off the soot. Aluminum anneals around 800 degrees.
The glue appears to stick directly to aluminum but generally this surface is also wire-brushed, then primed.
Wood
Marine plywood is preferred, due to its water-resistance.
Baltic birch is also occasionally used for inside pieces. Some of the new Finnish & Russian Birches are supposed to have exterior grade glues.
[The wood needs to be sealed on both sides with a varnish – usually we use Polycrylic or Varathane water-based varnishes.] Don't do this. The glue doesn't stick so well to Polycrylic. If you are doing two coats of glue, the varnish coat is a wasted step.
[The purpose of the varnish is to lessen the absorption of glue into the
surface of the wood and lessen the necessity for multiple coats of glue.] Ignore this comment.
Timing
It takes about an hour to do an adequate gluing – plan on an hour and a half
Don’t do too soon before lunch or quitting time.
You can do something else while waiting for the glue to dry – set a timer and clip the timer to your shirt.
If you let the surface get too dry, you will have to start over with the glue application.
Air Temperature – should be at least 65 degrees. 70 degrees is even better – set the heat up early so things can warm up.
Glue application
Two coats of glue are better than one. Put one coat on each side, let it dry, then do the second coat.
[The preferred application technique is to pour a manageable amount of glue onto the surface, then spread it out as quickly as possible.] Don't do this - the glue gets on too thick, and then doesn't cure properly.
Near the edges, use a flat brush technique - this will keep glue from slopping over onto the good side, or onto the floor or table.
The glue needs to be applied thickly and evenly.
Do the gluing where a reflected light can be seen in the surface – this makes it easier to see which areas are dry or not.
Be careful not to slop the paint onto the front of the metal or the wood – if this happens, wipe it up immediately with acetone – it is very hard to remove later when dried.
The edge of the surface you are applying the glue to should be hanging over free air – otherwise you will get glue on the table top or board, which will then transfer to the front of the piece. Usually this means setting the gluing surface up on 2x4s.
Put a lot of glue on the brush but be careful not to get it on the floor or table tops – wipe up immediately if it drips
How you know when the glue is ready
The glue should be tacky or almost dry – check every area
If it comes off on your fingers, it is not dry enough – you can use a heat gun to dry selected areas
You can see which areas aren’t dry yet – they will look shiny-wet in the reflected light and they will appear more yellow
The glue gets clearer as it dries
Using the heat gun
The Wagner heat gun is used to heat up the glue and to speed up the drying of the still-wet areas
You want to keep the gun moving – don’t overheat!
If the glue area is starting to bubble, you are overheating it. If it is turning brown or black, you have really cooked it and now it won't stick there.
Work on one surface, then work on the other
The metal surface can also be heated from underneath
Warm glue bonds better than cold glue. I always heat up both surfaces before the final joining.
Avoid over-drying the already dry areas
Silicone Paper – use at least two sheets
Silicone paper is used to position the two surfaces together
The glue sticks only very slightly, or not at all, to the silicone paper, so you can line up both edges and corners fairly accurately by touch
Then, holding one side of the metal firmly in place, lift up the other side, remove a sheet of silicone paper, then let the glue surfaces touch
press in place, then repeat on the other side
Hammering
Use rubber-covered hammer heads and deadblow hammers.
Make sure the front is free of wet glue. If you get wet glue on your hammer head, you will transfer that glue all over the front of your patina masterpiece. Then you will have to spend an hour cleaning it off with acetone
25 pounds per square inch is required to adequately compress the two glued surfaces together
a 2” diameter hammer has an area of 3.14 square inches and thus will require a hammering force of 78.5 pounds with each blow. This is achievable with a dead-blow hammer. A smaller hammer will require less force. Wear ear plugs if the noise bothers you.
use two hammers – one in each hand – this covers more area, more quickly and is easier on your elbows
Many of the copper pieces will still be warped or wrinkly – they will require more hammering to flatten. The contact cement will facilitate this. If the copper isn't too work hardened, you will be able to actually shrink bowled areas. This is the key reason why epoxy can't be used.
start your hammering in the center and work out to the edges in a star pattern – this will eliminate a trapped dome in the center
a small 1” hammer can be useful to flatten areas near brazing lines
check flatness with your fingers and keep working towards totally smooth flatness
Edges
Sometimes the edges don’t stick well, especially if the copper is too warpy or the glue was allowed to dry too much - in this case, the edges can be clamped
If wood is showing, let the glue dry an hour, then the wood needs to be trimmed off, and the edges sanded with 36 grit sandpaper on the hand-held belt sander
Curing time
Usually the piece needs to be allowed to set for 2-3 days before cutting can beginBacks
Backs are very important if you are gluing metal to wood. You need to have laminates applied to both sides of the wood.
The backs can be glued on as soon as the edges have been trimmed and sanded
Be sure to protect the patinated front – protect with mylar or cardboard or a piece of luaun.
Backs should be glued on as accurately as possible, but the coverage is not as critical as the fronts
Water and Heat Resistance
Appears to be satisfactory on outside designs in snow and rain. Appears to soften around 200-300 degrees. Copper outside in Arizona in direct sunlight might get that hot.
Copper glued to marine plywood lasts at least 6 months with total immersion in water (test not run longer than that).
Brands
I prefer the Weldwood-Dap contact cement in the red can, over the Liquid Nails version. The Liquid Nails version has less objectionable fumes, but greater water sensitivity. I am skeptical of the water-based contact cement. I have had a number of failures with it.
How to Repair Delaminations
This often happens when the design is waterjet cut at 60,000 psi.
Now you can use epoxy - I prefer 3M's DP-190 with the squeeze gun and special mixing nozzle. Be careful to clean any epoxy off the surface before claming. Use silicone paper and small blocks of luaun with your clamps to protect the patina.
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