Hotline Kiln Shelf Primer Review

One of the things that confused me when I first started working in fused glass was which kiln wash to use. So far I have used both Hotline Hi-Fire and Hotline Primo primer. This is what I have discovered about them.


The Hi-Fire primer, which has a pink color to it, goes on easier. It soaks into the shelf  and dries almost instantly. That means no waiting between coats. The Primo primer, which is purple takes longer to dry and seems to puddle. Because of this I have to wait a bit between coats.  The Primo primer also separates more and settles into a thick mud at the bottom of my jar. Each time I use it I have to make sure I get it mixed up really well before I use it. I just bought a wire whisk at the Dollar Store to help me mix it up. I had some marbles in the jar, but they just get stuck in the bottom.

Clean Up

Hi-Fire is a bit of a pain to clean off the shelf. I use a painter’s scraper to get the majority off and a green kitchen scouring pad to get the rest. Primo primer, as advertised, is easy to clean off. I just use the green kitchen scouring pad to rub off the old primer and follow with a damp paper towel to get the rest. In either case, I always wear a dust mask and rub gently to avoid sending the dust into the air.


Although both primers result in a smooth finish on the back of my pieces, the Primo primer was a bit smoother. To get the smoothest finish possible, I run the palm of my hand or a piece of rolled up pantyhose over the shelf before placing my pieces on it.


My conclusion is that each one has it place, and I will keep both in my studio. For my shelves, and molds I will mostly use the Hi-Fire primer. This primer will usually give me at least two firings on the shelf, and many, many, firings on the molds. Primo primer only gives me one firing. But because it is so easy to clean, I will use it for any molds that have more detail and also for castings. For example, my round pendants, like the one below, are made in a casting mold. In between firings I use a stiff toothbrush to clean off the old primer. I might also use Primo on the shelf if I am looking for an exceptionally smooth finish on the back.  I still have to do some experimenting on which I prefer for high fire projects like screen melts. Primo ads say you can use it for firings up to 1550 degrees Fahrenheit. I couldn’t find any information on how high you can go with Hi-Fire.

Of course, I haven’t tried Bullseye’s primer yet. Would love to know what others are using and why. Please leave a comment and let me know.

27 thoughts on “Hotline Kiln Shelf Primer Review

  1. wendy says:

    I wanted to know if you cure the shelf after you apply either primer? I know the instructions on the hi fire say to cure it, but isn’t that just to dry the primer?

    • Margot says:

      Wendy – The “curing” is just to remove all moisture. I usually prime two shelves at a time. One goes under my work bench and as long as it’s been several days before I use it I’m good to go. The other shelf I put in my kiln and fire afap to 250 F and hold for 5 minutes. That seems to be long enough to get the moisture out and still allow the shelf to cool in a reasonable amount of time so I can use it right away. I had an instructor once who didn’t even take the temp that high. His theory was that water boils at 212 degrees F so no need to go up to really high temperatures.

      I only use the high-fire on my shelves. I don’t use the primo often; just in certain molds where it would be difficult to remove primer. But in either case you want the molds dry before using them.

  2. Vera says:

    I used the hi fire was on a jewelry mold, cured it like the instructions state on the package, and all my glass stuck. What would cause that? I put on 4 layers – should I have not cured it? My firing schedule was 500 dph to 1500 and hold 20 minutes – was it too hot?

    • Margot says:

      Vera – I always cure after priming. Did the glass stick to the mold? Or did the primer stick to the back of the pendant? How many layers of primer did you apply to the mold? It could be that 1500 may have been too hot or too long of a hold. Are you using one of the jewelry pods that makes round or square pendants?

  3. Kristin Anderson says:

    Hi Margot, I use both like you do. I use the hi-fire on my shelves but actually recoat between each use. I don’t (even though recommended) scrape the shelf between each coat as I am afraid of making it uneven over time, so I just scrape it every approximately 4 coats. And then I use the primo on my molds and get more like 10 uses out of one coat.

    I did recently do a test and it seems like the Primo gives me a smoother finish on the bottom of a piece, but it also makes little shelf flaws more noticeable on the bottom of the piece. Because the hi-fire isn’t as smooth you don’t notice these little shelf flaws.

    I too have had a hard time on little jewelry molds with either of these two even though I cured and put many layers on. So I have started using the MR-97 boron on these molds with great success.


    • Margot says:

      Kristin – thanks for leaving your comment. I often wondered what happens if you just recoat without scraping. Sounds like it works. Does it make it harder or easier to scrape off when you do? I am now using 6 coats of the Hi-Fire primer on my shelves instead of the 8 to 10 coats that I first started out with! I can get two or three firings from this depending on the color of the glass and how high I am firing. It may be my imagination, but it seems like the old primer scrapes off easier when I use less coats. I switched to tile scrapper which is wider and seems to be little easier to work with than the paint scrapper. I don’t scrape too aggressively though. There is always a little primer left behind so I don’t worry about the shelf getting uneven. And yes! The boron spray is the way to go on some of those molds. I’ll be using it on all of my stainless molds in the future.

  4. Cori says:

    I too have used both of these brands. I switched to the hi-fire on the last primer I bought and just finished a 5lb container. I was thinking of switching back to Primo for the next batch because I get more firings out of the Primo and I didn’t have to remove the old wash before re-coating. I do remember not liking how long you had to wait to use the Primo after mixing initially. I remember I would have to think ahead and mix several hours before I was ready to apply so it was fully dissolved. I never had to cure the Primo like I do with the hi-fire. I have 20 shelves that I coat and I get about 10 firings out of the hi-fire and more with the Primo. Although I put WAY MORE of both the hi-fire and Primo primer on my shelves then you all are doing. I can not wash 20 shelves every other firing, that would take up way too much time! I coat it on thick, about 20 coats! I do not have a problem removing the old wash because I use an orbital sander with an 80 grit pad that is hooked up to a shop vac to reduce the dust. I also do my shelf clean up and coating on tables I set up outside so my shop doesn’t get completely covered in dust. Thanks for your discussion as I have been debating about whether I should go back to Primo or stick with hi-fire. One more thing, I have used several other shelf primers and this Hotline brand is the best! Every other brand would stick to the glass unless you got the thickness of the primer just right and once those other primers stick to the glass you cannot get it off unless you sandblast. I even tried using the kiln wash removing solution… doesn’t work!! At least with either of the Hotline products you can wash off any stuck shelf primer with a scouring pad.

    • Margot says:

      Cori – Thanks for this info. I am going to give the Primo primer another try. I only ever got one firing, but I have heard the formula was changed recently and some are getting more firings out of this new formulation. I am also trying Bullseye’s primer at the moment. I am having problems with one of their (expensive) kiln shelves. They recommended I use their primer. I just haven’t had the time to try it on that shelf yet. And you are so right! The primer removers never worked for me either.

    • herbert fellows says:

      20 coats??! If I use any more than maybe 6-8, I find the kiln wash flakes off when it dries. (hi-fire), Do you do anything special to get away with that? Different mix than recommended or ??

      • Margot says:

        I do not use that many coats. I only use 6 or 8. Six coats usually works just fine. If I am doing a low temperature firing I use 6 because I can then usually get another low temperature firing on the same shelf. By low temperature I mean up to about 1415 degrees F.

  5. Anneliese says:

    I once made a huge glass window that got stuck to the shelf. I finally got it off but the kiln wash was REALLY stuck. I simply soaked it and used a stainless steel scouring pad to get most of the primer of and then soaked it in toilet cleaner, (which I do after grinding my cabochons as well) and got every single piece of primer off without even having to re-fire the piece.

    I have just imported, (at great cost I must add), lots of little jewellery casting moulds, to South Africa, but I am petrified to use them as I live in a very humid coastal town and with my previous try even the Boron Nitride spray got the pieces stuck after 5 coatings and leaving the mould in the sun to dry for 25 minutes between coats. I have also imported the Hotline Primo and am going to try it when I have the courage. Please pray for me!

  6. oriel hicks says:

    I use Hi Fire primer but never use that many coats, and only take it up to about 180*C as that’s way above boiling point. The opaque glasses stick much worse than the transparent, but soaking in a bath of ordinary vinegar for an hour or two removes it entirely.

    • Margot says:

      It makes sense that you only have to go to boiling point. However, I vent my kiln and have noticed that at the boiling point temperature there still seems to be moist hot air coming out of the kiln. So I go a little higher. With the boiling point of water being 212 degrees Fahrenheit I now go to 300 degrees F and hold for 5 minutes. Some day I will have to ask Bullseye why they recommend going to 500 and holding.

  7. Leisa Clarke says:

    Hi Margot, I am using the primo primer having changed from bullseye. I am having significant issues with it sticking. My most recent three firings have all stuck to some degree. I have followed the instructions, fired before using and no luck.
    I need some ideas please as I have also had no luck removing the kiln wash without damaging the work! Leisa

    • Margot says:

      Hi Leisa. At the moment I use mostly the Bullseye or Hotline Hi Fire kiln wash. I just use the Primo primer only molds that have a lot of detail and reapply after each use. Are you firing on a kiln shelf? How hot are you going and for how long? What color glass are you using?

  8. herb fellows says:

    Has anyone tried using mr-97 bn spray as a shelf coating for firing?

    I do combing and have had varying degrees of success with Primo and Hi Fire, but haven’t found anything that works consistently.
    I thought I would give the bn a try, unless someone can tell me a definite no for some reason.


    • Margot says:

      I have only used MR-97 for slumping over stainless steel. It is my understanding that once you use boron nitrate on a mold you can no longer apply kiln wash to it. So be aware of that. Also, I am not sure it will hold up to the higher temperatures needed for combing. Let us know how your experiments go.

      • Edward Cantarella says:

        We(spouse and I) tried the ZYP boron, both spray and brush on. We felt in many cases it left a permanent haze on the glass. 409 cleaner allowed for several minutes removes ZYP pretty good (but not the hazing on the glass). The hazing was most noticeable on clear or light colored glass, like if slumping a clearish bottle. We eventually cleaned the boron off all the molds and vowed to never use again. When dry wo used Primo on them and there was no issue whatsoever. So we consider the idea that you can “never” apply kiln wash to a previously BN coated mold a bit “urban legend”.

        • Margot says:

          I only use the boron on my stainless steel molds. I also found it could leave a haze. Good to know the kiln wash worked after using the boron on a mold.

  9. Jo says:

    Used Hi-fire, 6 coats, fired the shelf and then fired a 9 x 9 sheet of glass painted with Glassline. First firing the shelf washcame off on the 9 x 9 sheet of glass. Fired second sheet, came out fine, fired a third sheet which shattered I to 1/2 bits. What am I doing wrong?

    • Margot says:

      Hard to know without more details. When you say you fired the shelf I am assuming you applied the kiln wash and then fired to cure at about 500 degrees for 15 or 20 minutes. You don’t want to go hotter than that for drying the shelf. What temp did you fire to on the 9 x 9. High temps can cause kiln wash to stick especially if it is white glass. As for the shattered piece that is another problem altogether. If the edges where it broke are smooth you most likely ramped up to quickly. If the edges are sharp than the cool down anneal cycle was too quick.

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