Replies

1. 
   SteveSchoene | Mar 11, 2007 02:53pm | #1

   You can spray varnish over it, but I don't recommend it.  Varnish takes so long to dry that when it is sprayed all the surroundings become coated with a sticky mess.  Varnish, including polyurethane varnish, is meant for brushing.  I assume we are talking about oil based varnish.  Traditional resin varnish (alkyd or phenolic) are less likely to have adhesion problems over other finishes than polyurethane varnishes though if the oil/varnish finish has had time to be mostly cured--say a month--the risk of problems would be diminished.  Polyurethane varnish doesn't provide much advantage over traditional resin varnish when the object isn't walked on with shoes.  The abrasion potential for most bookcases is moderate at most.  While books can move mountains, launch wars, and transport us to distant times and places, they don't often scratch the furniture. 

   There shouldn't be a problem spraying lacquer over the oil/varnish mix.  As always, more cured is better than less but its not as critical as it can be with some polyurethane varnishes. You won't get as much wear protection as with varnish, but lacquer (or shellac) should be fully adequate.  (Remember its only been roughly the last decade in which more durable finishes (conversion varnish or catalyzed lacquer) began appearing on furniture.    

    

1. 
   ajkelly | Mar 11, 2007 07:11pm | #2

   Thanks Steve.

   So, in your opinion, what can I do to make a thicker, glossier finish that is durable?

   I didn't mention my kids, did I?  So, you're right, those books probably won't scratch this bookcase, but my kids will try their best.  :)

   Should I just put on a coat (or two or three more) of the oil/varnish mix?  or just use a different varnish?  or spray lacquer?  or brush lacquer, for that matter?   I've read a lot of comments about woodworkers loving shellac - but I've never sprayed shellac.

   Also, I used some Paduak wood for the trim.  I know that it oxidize to a browner color - anybody have good ideas to seal/protect the wood to prevent (or retard) the oxidation?

   aj kelly

1. 
   SteveSchoene | Mar 12, 2007 12:01am | #4

   When I need a really durable surface I brush a traditional resin varnish such as Behlen Rockhard on relatively dark woods, or a Soya based alkyd resin varnish such as Pratt & Lambert 38 or McCloskey Heirloom when a lighter colored finish is called for.  For a wipe on finish, any of these works fine, or you can use something like Waterlox Original/Sealer which is a phenolic resin varnish already at wiping consistency.  It gives a nice mellow gloss that never looks plastic. (Even though phenolic resin is a plastic--formerly used in a number of products under the brand Bakelite.)     

   To preserve color of exotic woods, the most satisfactory way is to dye/stain it to match the colors you are looking for.  There is some loss of clarity, but with care it won't be too dramatic. 

   Edited 3/11/2007 5:04 pm ET by SteveSchoene

2. 
   Dan18700 | Mar 12, 2007 02:59am | #6

   Steve,

   I'm not sure I understand your answer.  I think the question is "Can I apply polyurethane over a varnish or oil finish?"  What is the answer to that?  There are times when this might be helpful.  The availability and attractive pricing of polyurethane would make it a useful option.

   Thanks, D. Gomez

    

1. 
   SteveSchoene | Mar 12, 2007 02:03pm | #9

   The question you pose is not one that can be answered with a simple yes or no.

   Polyurethane is just a variety of varnish, with a few advantages, mostly concerning abrasion.  The single part consumer grade polyurethane cheap and widely available, as you say, has significant disadvantages and represents another demonstration of the economic maxim "there is no free lunch".  One of those disadvantages is that it doesn't adhere as well to substrates made of other materials.  Shellac which still contains its wax presents a problem.  Oils or oil base varnishes or stains which are not almost completely cured will in some cases present a problem, though not universally. But, since you can't tell from labels which will and which won't have problems you might as well assume that adhesion might suffer unless you have waited a month or two to overcoat with polyurethane varnish.  Moreover, it is seldom a very important issue, since there are very, very few instances where polyurethane varnish will work where a traditional resin varnish will not work as well.  Floors may benefit from polyurethane; almost all furniture will not. 

   If using a cheap polyurethane varnish is important, you only need to avoid using it over other products that might offer questionable adhesion or you need to wait for oils or other varnishes to fully cure.  Only apply a product over other products specifically recommended by the manufacturer and you will be safe. 

   If at all possible it is always best, in terms of finish integrity, to use one top coat from start to finish.  Single part clear finishes are almost always their own best primer.  Using different materials under the topcoat finish are only desirable when there are specific needs.  The most common specific need is for coloration, and this may require a complex schedule of dyes, stains, toners and glazes.  If well chosen with reference to what the top coat will be these can be made to work compatibly  Sometimes dewaxed shellac can be a barrier between materials that might clash. 

1. 
   Rich14 | Mar 12, 2007 03:56pm | #10

   And there you have it. Use dewaxed shellac over the oil varnish, in order to apply any other top coat. The Shellac will solve your adhesion problems. Good old shellac. But wait! There's more! just keep applying shellac. Apply a 2#cut over the oil varnish. Let dry, level sand, lightly. Then another 2# cut. and either leave it alone, or level sand, then sub out as you like. All done with Shellac! Read any of the other threads raging here about Good Old Shellac. Wonderful. Hard! Durable! Perfect!. Rich

1. 
   SteveSchoene | Mar 12, 2007 04:48pm | #11

   Even though dewaxed shellac does a wonderful job of coping with incompatibilities it's still better to plan ahead and not have to use barrier coats and such.  Using consistent top coats materials that are compatible in the beginning with dyes and stains just has to be more long lived.  Different materials still do have different reactions to heat or moisture and I think those small difference which aren't apparent in the early years may eventually lead to an earlier finish failure than using as few different materials as possible.   Keeping the KISS principle in mind still may have value. 

   The easiest way to maintain such consistency is to use shellac as the top coat itself, not just for defining dye and separating stains or adding color. 

   Of course, this may be just the difference between deteriorating after 40 years compared to failure after 50, and we may well not care. 

2. 
   Rich14 | Mar 12, 2007 06:38pm | #12

   "Then another 2# cut. and either leave it alone, or level sand, then sub out as you like.

   Should have read, "rub it out as you like."

   Rich

2. 
   Dan18700 | Mar 12, 2007 08:49pm | #13

   Steve,

   Thanks for your comments.  I seem to recall reading somewhere that polyurethane was not to be used over certain types of varnish, but if one reads the container, or looks for instruction from the manufacturer it is not always readily forecoming.   For example, recently I tried to wipe some poly onto a plane handle that I though needed some finish.  However, it wouldn't dry, and that kind of spooked me.  The only thing I could think of as being wrong was that the underlying finish was not compatible.  On occasion, one wants to finish over an existing finish without stripping the old finish, but the type finish on the piece isn't always known.  I tend to like glossy finishes, and my purpose is often to give the finish more shine.

   Thanks, Dan

    

1. 
   user-83153 | Mar 12, 2007 09:28pm | #14

   "On occasion, one wants to finish over an existing finish without stripping the old finish, but the type finish on the piece isn't always known"

      

        This is never a good idea. Always know what the existing finish is before you attempt to go over it.  Unless you are working with an evaporative finish like shellac or laquer that will burn in to the previous coats you are asking for adhesion problems by refinishing without stripping. Especially if you don't even know what the existing finish is. Poly is the worst for this as it doesnt even like to stick to itself and has adhesion issues to begin with. One more in the long list of reasons why, unless you are finishing a floor, you shouldn't bother with poly.

                   Rob

2. 
   SteveSchoene | Mar 13, 2007 04:13am | #15

   A plane handle could have been rosewood, always a problematic, oily wood.  Oil from the wood mixes with oil from the varnish or oil/varnish making the combination very slow drying.  Handles may also have other contaminants from skin contact. 

   If you are prone to wanting to apply a glossy overcoat to an unknown substrate the safest choice is to stick with dewaxed shellac--it has about the best adhesion properties of any finish, is naturally glossy, and can be rubbed to really suburb gloss finishes.  It's a lot tougher than many seem to think. 

   Interesting enough, lots of polyurethane varnish labels warn not to apply it over shellac.  They say that because of shellac with wax.  Poly will adhere over dewaxed shellac. 

3. 
   ajkelly | Mar 13, 2007 07:37am | #16

   Lorne, Rich, Steve, et all;

   Thanks for all your advice.  I'm now a little wiser.

   I bought some Zisnner Seal-Coat Shellac today and some exterior-rated varnish and I can't wait to try it out.

   I'll let you know how it turns out.

   aj kelly

4. 
   Dan18700 | Mar 13, 2007 06:53pm | #17

   Steve,

   Harrah!!  You have made my day!  I can't tell you how much that bugged me.  You've done your good deed for today, and made a friend for life.  I'm going to tack up your bio so I can point to it and tell people, "There's the Man!".

   Thanks,  Dan

2. 
   lsteed | Mar 11, 2007 11:06pm | #3

   Hi AJ : I already posted a reply to your question about delaying the yellowing of paduak . In reply to this question I would suggest, since you have already finished the piece with the Behr product, to isolate that coating with two coats of ZINSSER SEAL-COAT, which is a wax free shellac that will adhere to the previous coating well and more importantly NOT promote any coating adhesion problems in subsequent coatings. Read my other posting (http://forums.taunton.com/fw-knots/messages?msg=35006.5) as to recommended coating system . In your case I would probably go for a UV rated varnish. Here in Canada I use General Paints exterior Woodcraft S/G Varnish but any good exterior rated varnish should do.

   Hope this helps!

   Lorne

   If you choose the brushable varnish system please use a good pro varnish brush such as the ox-hair brushes available from Purdy." if you are not sure if you have the time to do the job properly then don't bother starting it"

   
   

   ---

   Edited 3/11/2007 4:08 pm by lsteed

1. 
   ajkelly | Mar 12, 2007 02:33am | #5

   Lorne,

   Thanks for your advice.  I did read both threads - sorry for the dual threads running here.

   Your idea sounds like the best so far to me.  I'm going to try the Seal Coat and then a varnish over that.  I know that I can get the Seal-Coat down at Lowes or Home Depot.  I'll have to research where to get a good quality exterior varnish.

   With all these layers - my oil/varnish mix (which does state that it has UV blockers), a Seal-Coat shellac layer, and then some more varnish - I should get the best UV protection that I can provide.

   The lazy side of me keeps saying though, "why can't I just spray or brush lacquer or polyurethane right over the top?"  You're afraid that it won't bond well?  I'm counting on you seasoned pros to keep me from making that fatal mistake that forces me to strip the piece back to bare wood.

   aj kelly

1. 
   ajkelly | Mar 12, 2007 03:03am | #7

   Steve, Lorne, et all

   How about General Finishes Arm-R-Seal?  I was just digging in the shop and realized that I have some and I've never tried it.

   Any experience with or recommendations for Arm-R-Seal?

   aj kelly

1. 
   lsteed | Mar 12, 2007 06:06am | #8

   AJ : in answer to your questions about arm-r-seal I am not familiar with it. However the reason you should change the coating on your piece is that you have coated it with a drying oil product which will in fact hasten the yellowing of the substrate. All organic oils whether they are linseed, tung,walnut or any combination of them , will hasten the UV accelerated "aging" or darkening and yellowing of the wood. Heat added to the formula will further hasten the darkening effect. In other words a finished piece set in bright sunlit room in a hot climate will vastly age or darken quicker than a piece in a cool very darkly lit room. Oil finishes are by nature sensitive to UV rays and therefore age quicker than film producing finishes. In order to recoat your piece and ensure that you will have no finish recoating problems you should isolate the finish that is already there. Ideally new wood would be better, or in other words strip it. But, having said that, you should have no problems whatsoever by simply recoating your piece with Zinnser Seal-Coat first. It is important to track this particular shellac down as it is 100% wax free and there fore can be topcoated with pretty well ANY clear coat. Then you may proceed with the topcoat finish system of your choice INCL. urethane ( which is normally NOT compatible with shellac- which is why I asked you to use WAX FREE shellac) A lot of urethanes are very good resisting the effects of UV light- you will just have to check with the maker and read the specs to see. My suggestion of a pro grade UV inhibiting lacquer is still the preferred one but that is from a pro point of view AND only if you have the proper equipment to do so. Therefore ,if you do not have that type of set-up then we come back to the shellac-varnish system. Either way AJ in order to achieve your aim of lessening the darkening effects of UV on your substrate you have unfortunately started off with probably the worst product by using oils for that. Short of striping it and starting again then I recommend the shellac to isolate the coat you have and then choose either lacquer, exterior UV proof varnish or if you must then UV exterior rated urethane. Hope this helpsLorne" if you are not sure if you have the time to do the job properly then don't bother starting it"