Rendering over a Tar covered wall?

My friend wishes to Lime render her property but has found a substance on one of her walls which is like Tar. With this in mind can she Lime Render straight over it or will it require a sealant and if so what type.Rendering over a Tar covered wall?
She will have to fix chicken wire to the wall because I can't see render taking to the tar well if at all or take the tar off and I'm thinking the chicken wire would be easier.


http://www.green-trust.org/2003/ferrocem鈥?/a>


You can see in this picture using chicken wire to render over corrugated iron roofing, just screw the wire onto the wall using green plugs, nylon plugs and alike.Rendering over a Tar covered wall?
You can get a mesh made for this. Batten the wall with 30mm x 10mm 30 cm apart. Then horizontally 30cm apart. Use felt nails to fix mesh to battens. Apply first coat of render pushing it through the mesh. Leave rough then score it. Next day apply the finish coat, trowel over and when it firms up run a steel rule over to level of. When it starts to go hard use a Float for final finish.