HwndHost, DirectX 11 and WM_PAINT behaviour difference between Windows 7 and 8

I have an application that has a subclass of WPF HwndHost. This sub-class  overrides the HwndHost::BuildWindowCore method and creates a native HWND. I then create a D3D 11 swap chain using that native widow, draw some stuff into it and present it. As there is no animation in the DX image I am displaying I only re-render based on WM_PAINT messages I get by overriding the HwndHost::WndProc in my sub class

This all works as expected on Windows 8, I don't get many WM_PAINT message as I assume the DWM is using my back buffer to re-compose the screen as windows move around, get iconized etc...

However I then ported the program to Window 7 and the exact same code has different behavior. I get WM_PAINT message whenever any of my HWND gets exposed, say by dragging another window over it. I was assuming that the DWM, or whatever handles this for my on Windows 8 would do the same on Windos 7. So does anyone know if there is some magic I must perform to not get spammed with WM_PAINT on Windows 7?

The following shows some of the relevant code, BTW this is all in C++/CLI

    // Window creation in BuildWindowCore
    hWnd = CreateWindowEx(
        0,
        L"static",
        L"HWndHostEx",
        WS_CHILD | WS_VISIBLE,
        0, 0,                 // x, y
        300, 300,             // height, width
        parentHWnd,           // parent hwnd
        0,                    // hmenu
        0,                    // hinstance
        0 );                  // lparam

    // Swap chain creation
    DXGI_SWAP_CHAIN_DESC dxgiSwapChainDesc;

    dxgiSwapChainDesc.BufferCount = 2;
    dxgiSwapChainDesc.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;
    dxgiSwapChainDesc.BufferDesc.Width            = static_cast<UINT>( d3dViewPort.Width );
    dxgiSwapChainDesc.BufferDesc.Height           = static_cast<UINT>( d3dViewPort.Height );
    dxgiSwapChainDesc.BufferDesc.Format           = Color::getDXGIFormat( );
    dxgiSwapChainDesc.BufferDesc.Scaling          = DXGI_MODE_SCALING_UNSPECIFIED;
    dxgiSwapChainDesc.BufferDesc.ScanlineOrdering = DXGI_MODE_SCANLINE_ORDER_UNSPECIFIED;
    dxgiSwapChainDesc.BufferDesc.RefreshRate.Numerator   = 0;
    dxgiSwapChainDesc.BufferDesc.RefreshRate.Denominator = 1;
    dxgiSwapChainDesc.SampleDesc.Quality = 0;
    dxgiSwapChainDesc.SampleDesc.Count   = 1;
    dxgiSwapChainDesc.OutputWindow       = hWnd;
    dxgiSwapChainDesc.Windowed           = true;   
    dxgiSwapChainDesc.SwapEffect         = DXGI_SWAP_EFFECT_DISCARD;
    dxgiSwapChainDesc.Flags              = 0;

    dxgiSwapChain1 = dxDevice->CreateSwapChain1( dxgiSwapChainDesc );

Present Call

dxgiSwapChain1->Present( 1, 0 );

So basically 2 buffers in the swap chain and a synced Present call. The graphics cards are pretty similar, a Radeon HD 8700 on the Windows 7 box and an Radeon 8600 on the Windows 8.

Any suggestions gratefully received.