Number of polyurethane coats before I can polish?

[BishBashBosh]

Hi All,

My first post on UK Workshop so please forgive me in advance if my question has been answered before.

I am new to woodworking and have read a lot of information regarding working with polyurethane to finish a project. I have recently built a table top, which I intend to finish with poly, but cannot find anything concrete around the number of poly coats required before I can safely polish my table top.

I have purchased a gloss polyurethane and my aim is to finish my table top to closely resemble a glass look finish. The manufacturers guidelines recommend applying 3 to 4 coats of poly, which appears to be the norm, but I am worried 4 coats of poly will not be enough for me to then begin buffing/polishing to get that glass look finish.

Will 4 coats of ploy be enough or would I burn though these coats whilst polishing? Would anyone recommend applying more coats to be safe?

Thank you in advance.

 

[TheUnicorn]

I would do 4, sand with a fine grit and see how you get on, you can always put more coats

 

[mrpercysnodgrass]

The number of coats recommended is only a guide, you will need to keep applying until the grain is completely full. If your table top is beech you may only need two coats but if it is one of these asian mahogany lookalikes you may need four or five coats.
How do you intend to buff/polish to get that full gloss finish?

 

[profchris]

If you really want a glass look you will need to wet sand level once the finish has fully cured. Start at say 300 grit and work up to 1000 or 2000. Then buff. Automotive compound will get you a nice shine at that point, but full gloss will require further buffing with finer compounds.

If you do achieve a glass finish you won't want to use the table, of course, because the slightest mark or fingerprint will be glaringly visible!

 

[custard]

What polyurethane? Some of the water based ones are notoriously soft. You can get a rubbed gloss, but the surface will be too soft to hold it for very long. Micro scratches from household polishing means it'll end up satin. The irony is that because the surface is soft, even if you begin with a matt version, then the buffing effect of household polishing means that will also end up as a satin finish!

Oil based pu varnish will dry harder, but takes much longer to dry between coats. And you'll need a lot of coats. As Mr PS pointed out, that's partly due to how much the grain needs filling, but you'll also need a safety margin of extra coats so you won't rub right through and hit bare wood. In my experience you'll likely need 7-10 coats. Have you thought about about a two pack finish?

 

[Phil Pascoe]

https://www.rustins.ltd/rustins/our-products/indoor/plastic-coating-hardener-gloss

 

[Sgian Dubh]

One fairly common problem to be aware of if using the finish itself to fill grain, especially on coarse timbers such as oak ash, etc, is their tendency to eventually shrink to a greater or lesser extent into the sunken open pores of the wood. The process can take quite a long time to develop, even years to reveal itself, but a glass smooth surface created with this method may, in time, develop a certain amount of waviness or a rippled effect. It's just something to be aware of, but if grain filling is required to get a flat surface, another option might be to use stuff sold specifically for the job, e.g., a thixotropic grain filler, (there are other types too) which can be used on bare wood, or can be used after a first coat of sealing polish/finish. Slainte.

 

[johnnyb]

fill the grain with something. poly won't really work as its to soft. you need either a short oil varnish(yacht varnish is a long oil) a shellac(maybe brushing polish) varnish.(or French polish) cellulose or 2 pack lacquer.
the ones that form a single homogenous coat are best. ie short oil varnish remains as however many distinct coats so if you rub through one coat you will see the difference.
make no bones even rubbing out a sprayed finish using cured 2 pack lacquer on a small box is blooming hard work!(but gives a stunning finish)

 

[Jake]

I've got a thread somewhere on here a polished gloss finish using Rustins Plastic Coating (which is basically AC lacquer).