This excerpt deals with painting gourds for Purple Martin Houses, but the general idea works for all outdoor gourds. For gourds that will be used indoors, you can skip the sealing step and start with a latex or oil based primer.
Gourds should be painted before using for purple martin housing. But before painting, they should be treated with a preservative to help make them last longer. This can be accomplished by either brushing on Thompson's Water seal or a good dunking in a copper sulfate solution. Both will help preserve them and make them last much longer.
Here are the two methods in detail.
I do a lot of gourds, so what I do is pour a gallon of Thompson's in a large container. I use Thompson's because it's a good sealer and it's readily available in my area. Then, what I do is dip the gourd right down in the sealer and roll it around making sure I have good coverage both inside and out. I leave it sit for at least 5 minutes rolling it and moving it so the coverage is complete. Then, I remove it, let it drip dry for about a minute and then hang it somewhere out of the way. I let them set for at least a week, maybe two. I want the sealer to totally set in the walls of the gourds plus allow any excess to drain off.
Since this is part of my hobby and a winter project, I'm not in a hurry. Once, they are dried, I then proceed with the painting.
As for Copper Sulfate, it's pretty much the same method. however, it's much quicker. You'll need a large plastic garbage can for this. Mix one pound of copper sulfate for ever 5 gallons of water used. Don't over fill it or else it will spill out when you push a gourd down into it. Submerge each gourd for 10-20 minutes, holding them down with something heavy like a brick. (I use a long heavy stick and stick it into the entrance hole to hold them
down). Gourds are very porous and tend to want to float.
If your can is big enough, you may be able to get two or three in depending on the size of the gourds. Remove and drip dry, then let the gourds sit for about a day to make
sure they are fully dried on the outside. Good airflow expedites this.
The gourds are now ready to be painted. I strongly suggest painting them, because it will also help make them last longer. The outside should be painted either white or a very light pastel. Again two things. The light color
will help repel heat in the summer time and it will also help attract the martins. They seem to look for the light colored gourds and are drawn to the dark entrance holes.
First, a good coat of a good latex primer to seal the gourd and allow the paint to stick. Then, at least two coats of a good quality latex paint. I use white because it really helps keep the gourds cool in the summer. Then,
you'll have a gourd that will last for many seasons.
The following is an excerpt from an article written by Charles Myers in issue 7(1) of the Purple Martin Update, a magazine published by the Purple Martin Conservation Association. He describes his method for
painting gourds so the maximum life can be obtained from his gourds. This method uses a rubber like, weatherproof paint called Elastomeric Coating. Elastomeric Coating will last 3 to 5 times longer than paint, and will seal any small cracks that appear in your gourds. It's also known as "Kool Seal". It can be purchased from just about any major paint store. If they don't have it handy, they can order it.
Don't be in a hurry. Be sure and give all the steps plenty of time to dry properly.
Clean gourds with a bleach solution to remove dirt and mildew. Mix 1 part bleach to 10 parts water and use a medium bristled brush. After cleaning, let stand until fully dry. At this point, you can apply the waterproofing such as Thompson's or Copper Sulfate. Be sure and let stand until fully dry again.
Sand with medium grit sandpaper to rough up surfaces. 100 - 150 grit sandpaper works fine. Don't over do it and break through the hard outer skin of the gourd.
Prime areas to be painted, using a good quality Exterior Oil Based Primer. Let stand for at least 24 hours until dry. Clean up with mineral spirits.
Using a nylon brush, apply 1 to 3 coats of white Elastomeric Coating with one good coating being the absolute minimum. More is better than less. Apply straight from the can and do not thin. Elastomeric is easy to apply and will not run. Allow a minimum of 4 hours between coats.
If you can't find the Elastomeric Coating, then you can use a high quality, exterior gloss acrylic latex paint. Just be sure and use an oil based primer and let it fully dry before painting.
If your going to go through all this effort to put up your gourds, finish the job right so they will last.
Follow these instructions and you will have a finish on your gourds that will last for many years to come.
My thanks to Charles Myers and James R Hill for allowing me to use this excerpt from The Purple Martin Update.