How To Make Your Own Stretcher Boards Without Much Carpentry Knowledge

This is just one way to waterproof your boards.  There are many more suggestions on waterproofing boards online.  I'm hoping to share mine to add to your options.  

Materials you would need are:

• Marine Ply (plywood) of at least 1/4 inch thickness.  
• Sandpaper
• Plastic sheet locally known as mantel.  Available in most market places in the Philippines (dry goods/kitchen ware section.)  This plastic is a popular liner for picnic tables.  It may be possible to look for its equivalent in your area.  Clear plastic book covers are not recommended for this purpose as these do not have the give or stretch possible with the mantel kind.  Available in a wide range of colors and design.  I used green so you can see better.
• wide clear tape (at least 2 inches)
• cutter / scissors.

1.  Cut plywood to the desired size.  Use sand paper to smoothen the sides and especially the surface that you plan to use for the stretching.  Marine ply is a better version of plywood less prone to particles sticking out and falling off.  Still, you may have to pick a side with the least depressions and knots on it.  For very imperfect plywoods, you can apply masilya or filler coating and then sandpaper it smooth.  You do not need to apply paint or any other finish on the plywood.  Plywood shown here has been painted.  A previous attempt by me to follow the other suggestions online on how to paint seal your stretcher boards.  The problem there was I was not sure if the paint that I used was the correct one.  I noticed that the painted finish seems to take on surface dirt permanently.  So I went back to my previous method of covering the board with plastic.  Back to the project.  Once you've ascertained that the side you are planning to use is acceptable, proceed to the next step.

2.  Place your plywood on the plastic sheet and cut the plastic, leaving about 2 inches of allowance on all sides.  

Shown in the picture, plywood on top of mantel plastic

You can actually work with less allowance but for your first try, the two inches gives you better leverage to work with.  Check the cut plastic sheet for damages before you proceed to the next step.  Look out for holes and tears.  You can use either side of the plastic sheet.  

3.  Once the plastic sheet passes your inspection, lay it flat on the table and place the good side of your plywood facing the plastic sheet.  Use tape to anchor the four sides at the center of each side.  Imagine an equilateral cross configuration.  

To ensure smoothness on the finished side, when you work on one side, do the side opposite it next.  I colored and numbered the areas where I placed the clear tape.  (Clear tape was invisible on cam).  Do not scrimp on your tape but extend it at least 4 or five inches inward when you place.  The pull exerted back by the plastic may be enough to dislodge 1 or 2 inch tape strips.  

4.  Next, work on stretching the plastic edge adjacent to the already taped areas. In the illustration below, I used color coding and numbering to give you an idea of how to progress.  

You will probably notice that I seem to have gotten confused with my counting by starting at 3 and 4 but as I find it very funny myself, I left it as is.  Tape to the left and to the right (shown in orange) of each central axis point (cross area shown in red-orange).  Again, when you work on one side, work on the areas on the opposite side next.  Then proceed to the yellow areas and so on, until you get to the corner part.  

5.  The whole process really is about patience and controlling your strength.  The work is slow and methodical. When you get to the corner area, first tape one side that will go under.  You may have to tape it diagonally.  Important thing is, this should smoothen the plastic on that side of the board.  

Next, do a fold and then tape this over making sure to take up the slack left in that corner.  

Your finished stretcher board should look like this when you turn it over.

6.  Be sure to wash this new surface before use with soap and water to get rid of any oil that may have gotten on it from the market place or when you were working on it.  

Streching the plastic over the board this well takes a lot of practice.  Over time, you will develop a feel for the right amount of pull to keep the plastic taut but not take it past the breaking point.  So if at first, you find yourself having to redo the application over and over, do not be discouraged.  Balance the pull forces to get it just right.  

The advantage of using plastic sheeting include being able to use a sturdier support for your watercolors because you can use rigid plywood.  The cost is also much cheaper overall.  If I am not mistaken, a yard long length of plastic sheeting costs about 25 pesos.  The width of that is about 4 feet.  This also does not require extensive carpentry knowledge.  It also is easier to check the integrity of this board covering.  Just wet the surface.  Any breaks water can enter through will be marked because the wet spot underneath will be apparent on the plastic.  You can apply first aid to it by placing clear tape over the break. 

There are times when lack of options make using non-biodegradable materials necessary but we should minimize our carbon footrprint as much as possible.  Reduce wastage by re-using or recycling previously used plastic sheets.  After removing the paper and staples, tape over the holes punched by the staples and use the smaller undamaged inside area for another bout of stretching.  It helps also to have several plywood pieces in different sizes.  For new plastic sheets, I start out covering big plywood pieces.  Then as these get used, I would put the undamaged left over plastic on successively smaller boards.   Please re-use what can not be recycled.  

I hope you find this article and project useful.  Thank you for reading.  

This method is very labor intensive though.  If you have the dough, you might want to learn of other ready made alternatives sold commercially.  Read about one option by following the link below.
Gator Boards.

Other related articles in my blog:
Troubleshooting watercolor paper discoloration
Additional protection you can do yourself for your framed watercolor paintings
Understanding sizing: guide to wetting your paper
Paper stretching guide: understanding watercolor paper weights