What are the key differences between a water-based polyurethane and a oil-based polyurethane? How would one be better than the other? What lasts longer on wood, penetrates, Etc. When should there be one used over another (for a given project)?
Unfortunately this is an it depends.
There are some broad generalisations that can be made but waterbased polys have been steadily improved over the years (the early ones were pretty bad according to some accounts!) and now some have qualities that rival the oil-based counterparts. This, along with their inherent advantages, makes them well worth trying out when some years ago I would have recommended staying well away.
What are they?
First off before going any further it's important to describe what each of these actually is. Neither of these products is a polyurethane product in the way that you might be thinking, i.e. made from polyurethane only. Instead both are other coatings with some polyurethane added.
Waterbased polys are usually acrylic or co-polymer suspension coatings (not true varnishes).
Oil-based polys are modified alkyd varnishes, or uralkyds as they're described in the industry.
Knowing this goes some way to explaining why oil-based are generally superior and why some polys are much tougher than others, simply put they add very little to some versions and more to others.
The more they add the better the scratch-resistance, waterproofing and resistance to heat. So in no way can you think of "waterbased poly" and "oil-based polyurethane" as each being one uniform thing because they can vary enormously from brand to brand. But some broad generalisation can be made all the same.
What are they like?
Oil-based first since this came first:
- Naturally coloured, from a light yellow to something more amber.
- Because the oil base discolours over time these varnishes discolour, going slightly more yellow/ochre after a number of years (how much and how fast depends on total light exposure and the thickness of the application so normally there's no reason to panic that "Your varnish is going to turn yellow!").
- Noticeable chemical or solvent smell (depending on formulation). Not only is the smell strong to very strong it's also one that people often find offensive or nauseating.
- Odour dissipates slowly and is noted for lingering in enclosed spaces.
- Slow drying, relatively.
- Slow to fully cure, again relatively.
- Cleanup with mineral spirits (UK: white spirit). You can also use turpentine if you wish, but it doesn't make any difference to the varnish itself just to the user.
- Dilution with spirits. Oil-based varnishes can be thinned as much as wanted to provide anything from improved brushability (1:10) to a watery penetrating liquid (10:1) and anything in between. What's now called wiping varnish is a dilution of regular varnish of approximately 1:3 although you can go thinner or thicker as you prefer or as needed (some varnishes are thinner than others).
- Won't yellow.
- Lower odour (also a very different type of odour that people tend not to find offensive).
- Odour dissipates quickly upon drying.
- Fast drying to very fast drying.
- Fast curing, relatively. This process still takes much longer than the drying time, but is only a couple of days or so versus a week to 10 days.
- Water cleanup (although you want to use soap and water ideally, and warm water where available).
- Prone to bubbles drying in the finish. The product is inherently prone to bubble formation and the fast drying can easily lock tiny bubbles in place, where with oil-based varnishes the slower drying gives more time for bubbles to rise to the surface and burst.
- Not as easy to apply to a really good standard as the oil-based type, in part because of the fast drying and being prone to bubbles.
- Can't be diluted like oil-based can. A small amount of added water can be added if and when needed but the manufacturers and industry pros warn that over-thinning can introduce problems with film formation so it's generally not advised to thin this type of product to any great degree if you want it to provide good protection, and adding water can greatly increase the tendency for bubbles to form which is a huge headache.
- Can't be wiped on like oil-based poly. You can literally wipe it on, use a cloth to wipe the stuff on the surface, but it doesn't work at all the same way and the fast drying time means you can't wipe away the excess reliably even a short time after application.
You'll notice that I don't list above that they're colourless or "water clear" because that's no longer universal. Because the colour traditional varnishes impart is considered desirable to some users some tinted waterbased polys are now available.
Notes on drying
All the comments above about drying and curing times are based on average conditions. Where it's warm and dry everything dries and cures faster and people who live in such a climate tend to benefit greatly from this, although it can lead to frustrations working with fast-drying waterbased products.
Where it's cool and the humidity is high drying times can be greatly slowed and in the worst cases high humidity can prevent proper curing because the finish won't dry fully, resulting in a finish that's seemingly permanently sticky. Note that even if it's warm high humidity can retard proper drying.
The best of both types are excellent coatings, providing good to very good water-resistance and good to excellent scratch-resistance, but the worst of the waterbased type are still weak and not really worth using if you're hoping for good performance. So it's important to shop around and seek a product with good independent test results (can't go by what the maker says about the product!) and an abundance of good user feedback.
Some further reading from previous Q&As:
What is the difference between "curing" and "drying"?
What is the difference between wiping polyurethane and wiping varnish?
Can I switch from my oil base finish, that still has not dried, to a water based poly?
Can I apply water-based polyurethane over an oil based stain?
- Clear, non-yellowing
- Low(er) odor
- Cleanup with water and soap
- Applied with synthetic or natural brushes
- Amber/yellowing finish (darkens over time)
- Generally harder wearing
- Requires solvents to clean brushes; natural bristles recommended
- Longer recoat times
- Can easily be thinned with mineral spirits to make your own wipe on finish
Use oil-based if you want maximum durability and water resistance. Use water-based if you care about odor and want easy cleanup.