Why do we still have to use coasters to save indoor tables when there’s water sealant for decks that protects against rain and UV rays (albeit temporarily)?
In: Engineering
For the same reason why you can technically paint model airplanes with house paint, but you really shouldn’t.
Outdoor coatings are designed and formulated to work outside, usually on things like decks, railings and sidings, you know, big things which usually inherently have small variations or defects when inspected closely but sorta blend in and don’t become noticeable. Because of this, it’s acceptable for an outdoor sealant to maybe not have a surface finish that’s 100% consistent in colour or texture, might mute the wood grain a little or look kinda wonky in places cause it too blends in. Those sealants also usually need to be able to dry quickly and be super durable, which sometimes means they have to use chemicals or curing agents that might be fine outdoors because there’s constant fresh air, but become problematic in enclosed spaces, even if they do have ventilation, and if you’re buying a can of it, chances are you’re also applying it and therefore around for the curing process.
Indoor coatings, on the other hand, are much more delicate and consistent owing to the fact they’re usually going to be put on furniture that’s going to be held to higher scrutiny, be less likely to have defects (if you actually did a good job making it) and, well, not be exposed to the elements it’s whole life. They also take into consideration the fact that they’ll probably be applied indoors in a workshop or house, so although they say you need ventilation, they’re less likely to have super nasty chemicals and curing agents that overpower indoor ventilation. You’re probably also able to allow it to cure for longer without being touched, so it can level and smooth more than outdoor coatings which usually end up with leaves or people walking over it whilst it’s curing, so it’s gotta be quicker.
You can see this yourself, go and look at a piece of furniture treated with indoor sealant and really pay attention to it, especially the coating, and then go look at your deck with the same level of scrutiny and you’ll see colour variations, feel texture differences, notice that a few boards might curve a little, maybe even that the wood grain looks good in some spots but looks muted in others, then think about how long a deck still smells like the sealant after it’s been applied, like how you’ll still get tinges of it weeks later, then compare it to when you sealed a piece of indoor furniture and how it stopped smelling weird within days of applying the coating.
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