do we really need to seal the wood for oil painting?

Hello there,
The purpose of sealing the surface before painting isn’t just a mechanical safety against the effects of humidity or condensation making it move, it’s also to prevent a lot of colourful molecules from the natural surface (wood or cloth) from migrating through your pictural layer and staining it.
Golden has done extensive work on that aspect. I’d follow their advice. 

The issue with wood surfaces to paint on is that there are as many varieties as there are stars in the sky, each with its different risks or properties. Some might not require much precautions but as a rule of thumb, just seal them.
Acrylics in general are microporous but that varies depending on their polymer, formulation and drying conditions. This means that it will generally let humidity (gaz) flow through and the surface breathe, but will prevent water (liquid) from sipping through (with all it may carry back of tanins and stain from the surface). Gessos are meant to be more porous to give good ‘tooth’ to the next layer, so they are mechanically less effective as stain blockers.

The cheap option is to dilute slightly your gesso to give the first impregnation stain to the wood and when it’s properly dry to go for the next layer. (and then a third to correct remaining defects and smooth it over). That may work depending on the wood kind, the quality of your gesso and the quality of your priming work.
Cheers and good painting,
Lussh

Hi Lussh,

thanks for your comments. discoloration is not an issue for me as i never experience it and painting in oils doesnt get it affect anyway.
​what i concerns is in practice the water content in acrylic product wraps the wood, no matter it is called acrylic size, gloss medium or gesso, they all wrap the wood same way. so i not sure if they really seals the wood from water in following layer to wrap the wood further. 

And there many professionals had opposite saying, like one said wood had to be sealed before it can be applied any acrylic gesso ground. to seal it. however from Golden, they saying do not use any gloss medium under oil paints. 
so if we follow Golden’s advice, can acrylic gesso seals the wood same as “size” that prevent water content from following layer to wrap the wood?
hope experts like you can help to clear out. much appreciated. 

The topic of sealing wood panels for oil painting, especially when using acrylic gesso as a ground, can indeed be a bit confusing given the varying opinions and techniques available. Let’s break down the main points and best practices:
Why Seal Wood Panels?
When painting on engineered wood panels like Masonite tempered hardboard, sealing the wood is crucial to prevent warping and to protect the wood from absorbing moisture or oils over time. Unsealed wood can absorb water from the acrylic gesso, leading to swelling or warping. Additionally, wood’s natural acids can affect the longevity of your painting.
Sealing with Acrylic Products
You’re right to question whether acrylic mediums and gesso can effectively seal the wood, especially since they are porous:
Gloss Acrylic Medium as a Sealer: Some artists recommend using a gloss acrylic medium to “size” or seal the wood before applying gesso. While acrylic mediums are indeed somewhat porous, they can provide a preliminary barrier that reduces the absorption of water from subsequent layers. However, the water content in these mediums could still cause some degree of warping if not applied carefully.
Acrylic Gesso Direct Application: Golden and other manufacturers now suggest that applying multiple coats of acrylic gesso directly to the wood can be sufficient. They argue that the gesso itself can act both as a sealant and a ground, especially when applied in several layers. Each coat of gesso helps build a stronger, more water-resistant barrier, though initial layers might still cause slight wood movement.
Best Practices for Sealing Wood Panels
Given the mixed advice, here’s a recommended approach:
Initial Sealant Layer: Use a non-porous sealant designed for wood, such as a PVA (polyvinyl acetate) glue solution or a dedicated wood sealer. This layer will effectively block the wood’s pores and provide a stable surface.
Acrylic Gesso Application: After the sealant layer has dried thoroughly, apply 2-3 coats of acrylic gesso. This ensures a smooth, absorbent ground for your oil paints and additional protection against moisture.
Considerations from the Art World
AtPearl Lam Art Gallery​, where contemporary art often intersects with traditional techniques, artists frequently explore various materials and methods to achieve the best results. Their practice includes understanding the long-term effects of these materials. If you’re inspired by their approach, ensuring a proper seal on your wood panel before applying gesso can be a part of maintaining the integrity of your artwork.
I hope this clarifies the process and helps you in your artistic endeavors!