I have noticed that there is a bit of confusion here about the names and properties of the different surfaces available to stretch and support paper on. Though I don't claim to be an expert on watercolour, I owned a signage company for five years, and thought I would share my knowledge of potential backing/stretching materials with you. Click on links to see a picture of what it looks like. I hope this helps!
or foam board is a more or less paper product. Archival quality is available but ask before purchasing if this is important to you. It is extremely lightweight and prone to markings and punctures. We used it only for indoor purposes and temporary signage, and if it was going to be displayed in an area that was humid, or where it could get splashed, it would be covered with a sheet of vinyl sign film first. It is available in different colours and thicknesses, but most places only sell the white 3/16" width kind.
Corrugated plastic, also known by the brand name Coroplast
, or flute board, is an extremely versatile, extruded double-sided plastic sheeting. (Double sided meaning you can use both sides.) Coroplast is chemically inert (neutral pH.) It is durable, waterproof and archival and LIGHTWEIGHT. Corrugated Plastic was introduced to the sign industry as a substitute for poster board. It is generally available in 4 x 8 foot sheets, or cut to size. It is available in a variety of colours and in 1/8", 1/2" and 3/16" widths. Like corrugated cardboard (go look at a beer case!), corrugated plastic is aligned with flutes that run horizontally across the width of the surface. There is a slight detection of lines when looking at or feeling the surface. If you are working large, keep in mind the warp factor: somewhat flexible, Coroplast is not as rigid as some of the plastics that follow.
is a relatively inexpensive plastic derived from petroleum. Though we rarely used it, Polystyrene sheets are waterproof, smooth surfaced, lightweight and very strong, and made for great small signs when Coroplast was not a viable option because of its fluted grooves. CD cases are sometimes made from polystyrene. Remember those Super Slider Snow Skates? Polystyrene. The thicker widths break in a brittle fashion and are more difficult to trim to size than coroplast. It comes in white, black and a few primaries. I don't think I would personally recommend it for stretching paper as it is too bendy and flexible, and tape does not stick to it well.
or gatorfoam, is a type of foam board that is laminated on both sides with a wood fibre veneer. It is more durable than Coroplast in that its laminated surface resists dents and scratches better. Heavier than Coroplast sheeting, it is denser and less flexible than coro or styrene. Comes in various colours. We did not use this much, only 4 signs in 5 years. The only benefit it had against Plexiglas was that it didn't scratch as easily. But for stretching paper, it could be great!
is an acrylic sheeting, smooth and double sided. It has a shiny surface, and the thinner widths are relatively flexible. Comes in opaque or transparent colours
, and in several widths. Thinner widths are easily scored with a craft knife to break down to size. Waterproof and relatively lightweight. Watch those edges or you'll poke out a nostril. Subject to crazing and cracking in extreme temperatures. (We are talking extreme!) You can use a liquid plastic bonding solution to make sculptures or furniture with it. I made myself a red shelving unit out of it over ten years ago, and it still looks brand new. I use 8x10 clear sheets of the 1/8" thickness to stretch small pieces of watercolour paper to take on the road when I don't want to use a block.
is touted by some as bullet-proof and unbreakable. A product of GE, this impact resistant resin is strong, dude. It is more expensive than Plexiglas. My supplier had a display of a bullet embedded in a block of the substance; stopped by the sheer force of the lexan in a one foot solid block! Comes in white, clear, grayish and topaz, and is flame resistant. Probably too expensive, but if someone gives you a piece, give it a go!
Where can one get these materials?
Go to your local sign shop. Often they have parts of sheets leftover from other jobs that are in inventory that they'd be happy to sell you inexpensively. I know, because I did this for artists who came in to my shop! You can also ask to buy the vinyl sign film I mentioned, too.
If enough are interested, I would be happy to attempt a demonstration of how each of these materials measures up as a means for the stretching of watercolour paper, and compare these to using a wood board, if no one has done it here before of course.
Enjoy the day!
(edited for typo