So you have lumpy paint. Lumpy paint is a pain in the neck, and also fugly to take a picture of. (Bonus, if the lumps are pigment - mine were making nice blueberry smears until they were well-rolled-in.)
You can roll that lumpy paint on the wall, and when it's dry, use a low-abrasive sanding block, like a really old coarse grit one, to knock the bits off the wall, but don't rub too hard or you'll mess the otherwise smooth finish.
Still, I betcha don't want to roll lumpy paint on the wall. You'd rather filter it, right? But you either don't have a screen sieve of the right gauge, or you don't want to use one from your kitchen. So here's what you do:
All 10-pound potato sacks have a nylon screen window. If you don't have a 10-lb bag of potatoes, go buy one or ask your friend or neighbour for the bag. Do it.
- Cut the window out, leaving a good margin on the paper bag edges.
- Take some packing tape and tape the window around the top edge of the paint can, on the pouring side, of course, not with the handle in the middle. Don't tape it too hard because you might need to remove it after pouring. You don't even have to tape it at all, but it helps.
- Tuck the edges with tape or your fingers to make the window slightly cup.
- Pour the paint s l o w l y into the paint tray, through the nylon screen.
- If you're not emptying the can, then as you upright the can, remove the screen and pinch the corners together. Place the screen on the lid of the can or support it above the can so that it can drip. You can use your brush to encourage the lumps to give up the last of their good paint.
At the end of your painting, you can throw the screen out. In the environment, especially an aerobic environment (not a lot of landfills, though if the landfill is smart and harvests its methane, that's to be encouraged), the paper will biodegrade, and so will latex or linseed oil base paints - these will be digested by fungi and bacteria. Even petro-chemical-derived paints biodegrade, but it is certainly preferable that we return to using paints made in a traditional way. Also, keep in mind that biodegradation is a process that works on organic compounds; the minerals - like zinc and titanium - in the pigments stick around. And even if there was no other use for them than paint - which there very well could be - they can't be mined again. Which is why it's important to recycle old paint rather than throw it away (such as if you can't be bothered dealing with lumpy paint).
That means the only other thing that remains is the nylon screen, which is problematic, long-term. The thing that frustrates me the most is not that we use nylon screens for our food packaging - nets and screens are rather efficient - but that we use them to hold down mats of sod on slopes. So the sod that turns into a grassy, weedy hillside will be perpetually encased at the surface and root level by a nylon net. I bet that will be impossible to remove, let alone that it traps and maims a few field mice and other animals. Landscaping ought to be more ecological.