When splicing several narrow pieces of wood together to make one wider piece, such as in a firewall, it is very important to look at the end grain and alternate the grains accordingly, prior to gluing and clamping (see picture of 4' Jorgensen bar clamp I prefer), because if you do not, unless the grain is perfecly straight, your plank will surely begin to warp. It will be perfecly flat at first, but as the wood becomes exposed to humidity and temperature variations, in time, it is guaranteed to warp which will affect the position of your windshield and the fit of your hood (see diagram).
However, if you alternate the end grain of each piece, the individual pieces will still try to warp but the warpage will cancel out the warpage of the pieces to each side of it (see diagram). It may also, be a good idea to let the piece sit for several weeks prior to planing or sanding so it will become acclimatized to its' new shape and any small amount of warpage that occurs between the pieces can be smoothed out and corrected.
Also, try and get a board long enough to cut all the pieces from so the color and degree of warpage between the individual pieces will remain consistent throughout the plank...and get it slightly thicker than you need, for as you plane and sand out the warpage from both sides of the plank, it means that the finished plank will be thinner. than the original board.
High schools, community colleges or trade schools usually have a wide bed Professional "Powermatic" power planer through which you can pass your plank in order to plane the high points from both sides until perfectly flat. Do only 1/32" at a time so it doesn't chip.
Happy New year everyone.
Isn't that why plywood is better? Only glue is holding those boards together unless you add another board or piece of iron oriented 90 degrees to the wood grain. Not a bad idea to use biscuits as well.
Yes. Plywood is straighter (initially) and available in just about every type wood, but it is less durable than boards so is not necessarily better and not nearly so impressive and the individual layers will begin to swell and separate and become wavy if exposed to the elements and the exposed layered edges are not only unsightly, they are a freeway for moisture to the interior of the wood.
Plywood is made by turning the log and shaving off thin layers from the outside of the revolving log, so that the grain is not nearly as beautiful as the grain that can be found in quarter sawn boards or figured grained woods and for those who are after an outstanding job that will cause heads to turn, plywood is not the best choice. Joinery is more time consuming, but, if done right, the results are well worth it. Please see the below link for a quick lesson on the various types of wood grains. Jim Patrick
PS. The boards should be joined using waterproof glue such as 2 part Weldwood resourcinal glue used in marine and boat making applications.
The 16 orignial firewall I rebuilt used a continious dovetale joint between each board locking them together, then a layer of vaneer was applied to each side, I ran the delaminated vaneer off the core in my plainer. The dovetale boards were reglued then a layer of white oak vaneer was glued to each side. Guess some of them used plywood depending on who supplyied them. Its not the correct hard rock maple on the vaneer-- but I wont tell anyone!
That's OK Paul. After you paint it black, as it was originally, then no one will know.
Sorry Mike-- it will stay naturial! just like the hunk of ship lap dashboard I could do much better setting on the windshield! There is a floor dimmer switch installed on it----------dont have a clue what it was used for but going to have fun making it work something!
Thanks Jim. Good to know this stuff.
Good advise Jim, I have used that method for years on table tops with good results. Like Dave, I also use biscuits in all the glue joints. I find it works best to cut each board no more than 4" wide, never had a panel warp. Thanks, Rick
Thanks Jim for the tips on firewall reconstruction.
Your suggestion to alternate the grain in the boards to reduce warping is welcome. Your drawings show half sawn boards which will naturally cup. If possible, it is even better to use quarter sawn boards. With these the growth rings/grain is across the board and warpage is almost negligible.
Can you guys still buy sliced veneer to finish off the job? I cannot find a supplier here. All we can get is rotary cut veneer and the grain shown is nowhere near as nice as the natural appearance of sliced veneer.
Consequently, I use 3/4" MARINE GRADE ply to cut firewalls. This shows the rotary cut outside veneer, the same appearance as I could achieve using boards and gluing on my own rotary cut veneer, without all the hassle. I have had to refinish the dash in my 1912 van once in 15 years and it is due again now, not because of de-lamination, but the breakdown over time in the polyurethane clear varnish.
Allan from down under
Try "Constantines", www.constantines.com. They are a good source for woodworking tools, products, stains, exotic woods and veneers. They supply fantastic waterbased wood stains that arrive in powdered form for you to mix. I much prefer waterbased stains over oil based, since the wood absorbs the water based stains much more deeply into the wood giving it a 3D appearance. The colors are also much more vivid, accurate and available in a wider variety and more easily to control by either applying more stain (darker) or applying more water (lighter). Only downside with waterbased stains is that they can cause any loose wood grains to stand up.
Remedy: After sanding and prior to applying the stain one should wipe down the piece with water and let it dry. this will cause any loose grains to stand up and dry in the standing position, which you can sand off with a new (not used) piece of 320 sandpaper. Use new sandpaper to cut the grains off with a few swipes. Used paper will not cut them, but press them back down. If you skip this step the water based stain will cause the grain to stand up and in sanding the grains off after staining will leave you with a discolored surface.
After the waterbased stain drys you can see what it will look by wiping down with mineral spirits. If it is too dark, after the mineral spirits has evaporated, you can make it lighter by wiping with a water dampened rag, or if it is too light, you can apply more stain. When you get it where you want it, coat with satin Polyurethane. Jim Patrick