@njnyjobs
Because paint isn’t that expensive – people often give it away for free on Craigslist or Yahoo Freecycle – and because priming, painting, sealing will make it protect the wood a lot longer so they don’t have to repaint frequently, deal with flaking paint (which I consider a pollutant), and/or have their wood rot so that they have to rebuild in a few years.
If you just apply paint without primer (to wood), first the wood would absorb some of it, so you’d end up using more paint. Second, paint doesn’t bond all that well to pure wood. The primer creates a surface that the paint can adhere to. Without it, the paint will fail (come off) much faster – again, resulting in you having to use more paint. Sealing again just prolongs the life of your paint job. Nothing will last forever, but if you maintain the sealant coat, you’ll probably never have to paint again. Of course you can always skip the sealant and just plan on repainting from time to time. Now that I think of it, I don’t think I’ve ever used sealant on wood – only metal. You could also just go with the sealant and skip the paint…
I don’t see how tar paper/vapor barrier sheets + R31 insulation (which is a lot by the way – the insulation you get from the added material after R19 almost always does not warrant the extra material or cost) + cedar plank shingles is going to come out cheaper than paint! It most likely will not. It definitely will not where I live…
And then there is the added issue of which side to apply the vapor barrier to, which depends on where you live. And if the insulation were to ever get wet (how tight can you make the shingles, what is the humidity where this shed is, will they maintain it to prevent leakage), it would be useless and may start molding.
@squirbel Don’t forget to make sure that the paint is made for wood (not metal) and if you get a sealant make sure it works with the paint you get. I’ve never used both sealant and paint together. I’ve always used either primer + paint or just sealant…