I would like to know also!
Dawn

I prefer poly thread. It is stronger and does not fade over time. The newer ones have a better sheen to them. Never had a problems with breakage or fraying. I also use florist netting to cover them. You can buy them or ask your supermarket florist to save them for you free of charge
Sukiray.

My collection of poly embroidery colours is growing so I use the rayon that I have just for the colour and on Items that will not be washed extensively.
The orange of the rayon thread has run and spoilt a baby fishes quilt,lucky it was used only in one place.
I won't be buying any more Rayon thread.
I'm doing a lot of table cloths and pillow cases so I will definately only use Poly Embroidery on them.
Happy stitching .
From Bev

Thank you for asking this question. I am waiting for all the comments.. Hugs sally

An interesting question and thanks to all for their answers

I use poly thread. I tried rayon thread on a design mixed with Poly. After washing and bleach the poly thread stayed vibrant and the rayon thread faded so badly I decided to only use the poly thred from now on. Hope that helps you out. Suzanne

I use both but rayon breaks more often. I have never had any to bleed.

I think it depends on the look you are going for. Rayon is more susceptable to bleach. Rayon has a higher sheen, so in certain projects, it may be preferred. If an item is going to be washed frequently, use polyester. I've been doing a lot of FSL, and I like the look of rayon, so I've been using rayon. But since you can mix the threads in the same design, use whatever you have on hand. you might want to test the bright rayon colors for color fastness before using.

I use rayon and have never had a problem with fading,running, etc. I do not use bleach on things that have been embroided anyway. I only use Robison Anton rayon and have been very, very, happy with it.
Carol

I use "everything"....rayon, polyester, cotton...depending upon the project and its properties and what it will be used for.
Polyester is toughest, so therefore used when the article is to be laundered often and requires for tough wearing...and is appropriate to the fabric upon which you are embroidering.
Rayon is "softer"...appropriate if this look is required for blending colours. Also, if you are making 3 dimensional flowers where the fabric is polyester organza and you plan to cutaway the surplus fabric outside the embroidery with a heat tool (to seal the edges) you must use rayon as it will not melt against the heat tool tip. (Makes great 3D flowers!)
Cotton ... on heirloom work,(think, fine weave voile etc) or if you are doing something requiring a crochet look finish as in doilies etc.... used in conjunction with washaway or heataway stabilisers. A fine cotton thread looks best here..mimics hand crochet look. Hope you find this post useful. Cheers, Mufti