The pandanus tree has a lot of "prop roots" supporting the trunk. Look among the roots. You'll see a lot of things that look like paintbrushes without handles. Gather a whole lot of them and use them for that.
That's what islanders do wherever this tree grows. I first encountered this trick in the Marshall Islands. A walk on the shore there will reveal thousands of brushes with soft silky bristles. Locals told me to make sure the brushes are clean, since they are also used for toilet paper there.
Step 1: Find a Pandanus Tree
Here's what it looks like. The tree is called "screw pine" by obsolete botanists because the leaves grow from the trunk in a helix. Hawaiians made sails and sleeping mats by braiding the leaves. You often hear the Hawaiian word "Lauhala" to refer to any matting. Hawaiian "Lauhala", hala = leaves, lau = pandanus. Watch out for the teeth on the edges and center spine of the leaves.
Some varieties of pandanus are bred for especially long strong leaves, just for making matting. Some are sterile and have no fruit. Those are grown from cuttings.
In Micronesia they've bred some varieties with huge fruit. I've seen some bigger than the biggest watermelon. I've heard they can weigh more than 100 lbs.
I picked up a bunch of old keys under this tree by the Mo'olele canoe shed in Lahaina, Maui.
Step 2: The Fresh Fruit
The fruit is a big thing that looks like a giant pineapple without the leaves. It comes apart into sections called "keys". Chew on the orange part and suck the juice from them. It's full of fibers that are firmly attached to the base. That will be your paintbrush later on. The fruit is sweet and tastes like mango plus cantaloupe, but I don't like it. I don't like it a lot, because in Majuro, Marshall Islands, I got really sick after eating them. Probably a coincidence, but food aversions don't care about logic.
Another traditional method is to roast them, pound them and wring out the juice to dry into a sort of fuit leather.
Step 3: Card the Bristles
Make a handle by screwing a big drywall screw into the base.
Rub the bristles on a wire brush to soften them up and dislodge any loose ones.
The best paintbrushes come from keys that get beaten by the surf and then wash up. Watch out for sand that comes out of the inner part of the key. A trip through a washing machine would probably be good for the brushes regardless of origin.
Step 4: Nature's Disposable Paintbrush
I especially like these brushes for epoxy glue.
I feel bad about throwing away a commercial brush every time I glue something.