So I've spent a couple of hours chatting with another artist about how certain artistic methods are often taken for granted by those who routinely use them, since they've forgotten the expensive trial-and-error process they had to go through. For me, stretching paper had, perhaps, a 30% fail rate. It wastes time and, in the case of thinner papers, can waste materials, too.
Whilst training at college and, for a while, whilst illustrating commercially, I used to use a thick watercolour paper, Arches 140lb hot-pressed. It takes a lot of abuse, can be sanded, scraped and soaked. And, if you really balls something up, you can shower off your gouache painting and start again. That last one's not so good for watercolour and is no good for acrylic.
My current project is self-funded. It's also pretty big and, being a book, there's no remuneration until everything's done. So I've decided to work with 200gsm Fabriano cartridge paper. It's thick enough to take decent paint layers and thin enough to put through a laser printer - and works out at 14p per sheet (420 x 297mm) - versus two or three quid for the equivalent size in Arches.
As with Arches, cartridge paper really ought to be stretched prior to painting lest it 'cockles', meaning that the paper's surface becomes uneven and doesn't dry flat. Stretching paper is a way of loading the paper with water so that it expands evenly. It is then secured to another surface and dried. The water evaporates but because the paper is secured, it cannot shrink, and remains at its expanded size and under tension. The addition of paint doesn't not cause the paper to expand, because it cannot expand any more. When the painting is complete (and dry), it is removed from the surface and remains smooth and flat. The following is my way of doing it. Other people use different methods so find the one which works for you.
Wooden board: something like a 3-ply, or MDF, big enough to take your paper with a couple of extra inches all the way round.
Gumstrip sealing tape. I use a brand called Butterfly, manufactured by Wiggins Teape (Stationery) Ltd. (Yes, it does say 'Teape' and not tape).
1. Cut some appropriate lengths of gumstrip, two for the lengths of your paper and two for the widths. Make it an inch or so longer than the dimensions of the paper.
NB: Keep your tape in a plastic sandwich bag so that it can't get wet! A wet roll of gumstrip is a DEAD roll of gumstrip. Grrr!)
2. Fill your bath, or suitable container, with an inch or two of cold water. Obviously your container needs to be large enough to take your paper.
3. Carefully float the paper on the surface of the paper. Try not to allow water to spill over the surface. A few drops ain't the end o' the world, but we're trying to avoid submersion of the paper. (Arches and thicker papers can be submerged, but this is thinner paper and may split if over-soaked.)
4. After five minutes or so, carefully lift the paper off the water's surface, and dangle it until the water stops dripping off it. Then lay it on to the board, roughly centrally.
5. There are right and wrong ways to apply the tape. DON'T chuck all your tape in the bath. Its adhesive will soak off. DO apply water quickly to the tape quickly, preferably squeegeeing off the water almost as soon as it's on. Lay the tape on the paper, so that approximately half the tape is over the paper. Run your hand/finger/approximately-similar appendage the length of the tape, to squeeze out any excess water which may delay drying. The longer this entire set-up is wet, the more likely it is to fail.
6. Take a clean towel - preferably one which doesn't shed fluff all over the place. Lay it out on the board and paper, and pat it down to soak up excess water. Then, lay the board out flat where it can dry.
There you go. Hope it works for you.